killing babies hrdy on the evolution of infanticide

8/12/2019 Killing Babies Hrdy on the Evolution of Infanticide

1/32

This article is a preprint of an article published inBiology and Philosophy,20: 271-289

Springer 2005. The final version of the article may be found at

http://www.springerlink.com/content/72040j322w1l0502/?p=72ccd95a29c9482ea5497a95db951

9f3&pi=3

Killing Babies: Hrdy on the Evolution of Infanticide

Catherine Driscoll

Dept. of Philosophy

Dartmouth College

6035 Thornton Hall

Hanover, NH 03755

email: [email protected]

tel: (603) 646 2112
http://www.springerlink.com/content/72040j322w1l0502/?p=72ccd95a29c9482ea5497a95db9519f3&pi=3http://www.springerlink.com/content/72040j322w1l0502/?p=72ccd95a29c9482ea5497a95db9519f3&pi=3http://www.springerlink.com/content/72040j322w1l0502/?p=72ccd95a29c9482ea5497a95db9519f3&pi=3mailto:[email protected]:[email protected]:[email protected]://www.springerlink.com/content/72040j322w1l0502/?p=72ccd95a29c9482ea5497a95db9519f3&pi=3http://www.springerlink.com/content/72040j322w1l0502/?p=72ccd95a29c9482ea5497a95db9519f3&pi=3


2/32

Catherine Driscoll/ Killing Babies/ 2


Abstract

Sarah Hrdy argues that women 1) possess a reproductive behavioral strategy including

infanticide, 2) that this strategy is an adaptation and 3) arose as a response to stresses mothers

faced with the agrarian revolution. I argue that while psychopathological and cultural

evolutionary accounts for Hrdys data fail, her suggested psychological architecture for the

strategy suggests that the behavior she describes is really only the consequence of the operation

of practical reasoning mechanism(s) and consequently there is no reproductive strategy

including infanticide as such, nor could the alleged strategy be sufficiently mosaic to count as an

adaptation. What might count as an adaptation is a window before bonding that permits

practical reasoning about the reproductive value of infants and hence variable maternal

investment, and which, contra 3) arose early in hominid history due to a combination of

increases in infant dependency and increased human abilities for conditional practical reasoning.

Keywords: cultural evolution, infanticide, psychological adaptations, Sarah Hrdy, sociobiology.


3/32



1. IntroductionAlthough the idea that women may be equipped to kill their own children under certain

circumstances may seem horrifying to people who are educated to believe that the natural state

of a mother is to love and raise her children at all costs, Sarah Hrdy in her bookMother Nature

(1999) argues that this is indeed the real character of the maternal instinct. Infanticide is

extremely common in both western and non-western societies, much more common than

westerners are willing to believe.

Hrdy argues that the adaptiveness of the conditions under which infanticide

1

occurs and

its universality suggests that it is part of a larger reproductive strategy that is an adaptation.

Infanticide is usually induced in women in very difficult social and economic circumstances.

Superficially at least, killing or abandoning one's own children appears to be maladaptive - such

behavior appears to reduce the number of children a woman has and therefore her fitness.

Nevertheless, Hrdy claims theprima faciebelief that infanticide is maladaptive is mistaken,

since a woman's reproductive success is not to be equated straightforwardly with the number of

children to which she gives birth but rather with the number of children that she is able to raise to

sexual maturity. Women face different reproductive challenges from men, in that they are

limited in the number of children to which they can give birth. Hrdy thinks this has promoted a

reproductive strategy in which women try to maximize quality rather than quantity of offspring.

Infanticide, as a part of this strategy, is a response to circumstances; in particular it is brought on

by a woman's lack of preparation for motherhood, the poor health of a child to which she has

recently given birth or the lack of appropriate resources - be they material or social - which she

needs to successfully raise her children.


4/32


To briefly summarize, Hrdy is offering evidence in support of the following hypotheses:

1. Women possess a conditional behavioral strategy (which I shall term the

female reproductive strategy including infanticide, or FRSi), which allows them to vary

the level of investment they make in their offspring up to and including infanticide and

abandonment. This strategy is in some sense innate or biological.

2. The female reproductive strategy is an adaptation; it is present in the

human population due to the action of natural selection, and because it maximizes a

mothers fitness under a wide variety of circumstances.

3. The female reproductive strategy arose in response to a particular suite of

human ecological circumstances:

a) In the last 10,000 years

b) As a response to close birth spacing brought on by the advent of

agriculture.

This paper is designed as a critique of Hrdy's work; in it I intend to show that there are

serious problems with the evidence she presents in favor of her three hypotheses. First, I intend

to show that it is unlikely that women possess a reproductive behavioral adaptation of the sort

Hrdy has in mind. The architecture Hrdy suggests as the substrate for the FRSi renders it

unlikely that it could count as an adaptation in its own right, even if there are capacities that

might permit conditional investment and those capacities might be adaptations. Third, I intend to

show that those capacities that permit conditional investment, if they are adaptations, are not

likely to be adaptations to conditions following the agrarian revolution; instead they are more

likely to have become adaptive with the onset of new reproductive challenges faced by mothers

once human infants began to have much longer childhoods and require more resources to be


5/32


raised successfully.

2. Hrdys first two hypothesesIn this section I am going to deal with Hrdys first two hypotheses. First I am going to

consider in detail what Hrdy takes to be the nature of and evidence for the existence of a specific,

universal FRSi, and what Hrdy might mean when she says that the FRSi is innate or biological.

Section 2.3 will examine evidence for the claim that the FRSi is an adaptation. Finally I am

going to consider some possible alternative explanations for Hrdys data other than the existence

of an adaptive FRSi. It is possible that the pattern of conditional maternal investment which

Hrdy describes can be accounted for as the consequence of a psychological disorder, or by

cultural evolution, or as the consequence of ordinary practical reasoning, rather than as part of a

larger reproductive behavioral strategy.

2.1. What is the FRSi?

Hrdy is not entirely clear about what she thinks is the structure or substrate of the

reproductive strategy, but it seems that she has something like what follows in mind. The FRSi

is essentially a conditional behavioral strategy that governs the degree of investment that a

mother makes in her children, especially new infants. Human mothers that possess this strategy

vary their level of investment in their infants depending on their personal, familial, social and

economic circumstances and the relative health of their new infants. The responses generated by

the FRSi reflect the constraints of womens reproductive biology: Women are only capable of

carrying one infant every nine months, and have to then make the very large post natal

investment of lactation and other forms of care. In practice, a woman can only breastfeed one


6/32


infant at a time and in hunter-gatherer societies women can only safely manage one baby every

three or four years (ibid.: 200-204). Since a womans genetic survival depends on these very few

offspring, it is much more important that a woman pursue quality of offspring over quantity.

Hrdy speaks as if women actually do this by (whether consciously or unconsciously) weighing

up the relative benefits of keeping or rejecting a child for their overall reproductive success. In

essence, mothers are attempting to raise as many children as they can to adulthood; however, this

does not mean that mothers will attempt to raise every child to which they give birth. Instead,

they will make choices about which children to keep and which to abandon. Mothers can engage

in different degrees of care, from complete investment to infanticide or abandonment.

Much of Hrdys evidence for the unusual conditional nature of maternal investment in

humans comes out of her discussion of the difference between mothering in humans and other

primates. Primate mothers almost never kill their own babies (even if they do kill the babies of

others) nor differentiate between them in the degree of quantity and quality of care. Human

mothers, however, do make these sorts of differentiations. Hrdy suggests that the change in

between primate and human mothers is determined by a difference in the patterns of bonding that

take place in humans. Primate mothers usually bond immediately and permanently with their

offspring (ibid.:178-179). In human mothers, however, there is a delay in bonding. Women

usually do bond with their children, but this does not always happen immediately after birth

(ibid.: 166). Hrdy suggests that there may be a short-ish window of time (approximately

seventy-two hours, ibid.: 538) in which a mother is ambivalent towards her baby; in fact, that it

takes spending time and associating with the child to allow the mother to properly form an

attachment to it. Human mothers do not often feel strongly attached to their children until breast

feeding has begun and with it the production of hormones that facilitate emotional attachment


7/32


(ibid.: 137-141, 535-539). This period of time can be lengthened if the mother is removed from

the child and breastfeeding and other behavior likely to create attachment does not take place.

Hrdy suggests that a failure of attachment to babies is part of the cause of infanticide or

abandonment. Hrdy cites Fuchs's (1987) description of an unintended experiment in 19th

century France in which destitute mothers were forced to remain with their babies until eight

days after birth. The rate of abandonment dropped from 24 percent to 10 percent (Hrdy, 1999:

315). The growing attachment that a mother undergoes with contact and bonding with her baby

make it much less likely that a mother can choose on a rational basis to abandon or keep her

baby (ibid.: 316).

Essentially, it is this change in the pattern of bonding that permits human mothers to

engage in conditional investment. The bonding that takes place in primates effectively acts as a

fast and dirty way of getting primate mothers to do what they need to do without having them

decide whether to look after their child. Human mothers, however, have conditional and

practical reasoning abilities that permit them to adjust their level of investment in their infants in

the light of the effects on their own future reproductive prospects, the needs of any current

children in whom they have already invested, and the potential of the new infant to be raised

successfully. The combination of capacities for conditional practical reasoning and the bonding

delay gives human mothers the capacity to respond to changes in their environment by changing

their patterns of mothering.

2.1.1. Is the FRSi innate?One claim we need to address is how far Hrdy thinks the FRSi is innate or biological

(her term ibid.: p. 56-7). The notion of an innate trait (understood to mean a trait that isnt


8/32


learnt) is problematic, because no trait develops in an individual without the interaction of a

wide variety of developmental and environmental conditions, often including learning. Hrdy is

sensitive to this fact - in saying that maternal reproductive strategies (and that includes human

reproductive strategies) are biological, she certainly does not mean that somehow genes directly

control maternal behavior indeed Hrdy resists this claim even for species of honey bees and fig

wasps, whose conditional maternal nurturing behavior is determined by a complex variety of

developmental and environmental effects, along with genetic dispositions (ibid: p.59-66).

Human behavior is hardly likely to be less environmentally sensitive. However, there are two

common components of the innateness concept that Hrdy does seem to want to ascribe to

womens reproductive behavior. The first is that the FRSi is highly heritable. Hrdy does seem

to think that womens reproductive strategies have been acted on by natural selection, and hence

need to be reliably heritable across many environments. Her discussion also suggests that the

FRSi is universalor nearly universal (the exception being, due to her third hypothesis, women

who belong to groups that have never had any history of agriculture).

2.1. The FRSi is an adaptationThe second of Hrdys hypotheses is that the FRSi is an adaptation. An adaptation is,

roughly speaking, a trait that is present in a population because it was selected for under the

conditions which existed in the ancestral population. The claim that something is an adaptation

is not equivalent to the claim that it is adaptive; an adaptation need not be something that is

adaptive at the present time, since the environmental conditions that existed at the time of the

evolution of a trait may not be those that hold in the current environment. The primary way of

illustrating that some trait T is an adaptation is to show that T is optimal or fitness maximizing


9/32


for the organism that possesses T in those environments in which T initially evolved. In this case,

that would mean Hrdy has to demonstrate that an FRSi as she describes it would have been

adaptive in the maternal EEA (environment of evolutionary adaptation). However, merely

demonstrating that T would have been optimal or adaptive is not enough to demonstrate that T is

an adaptation; one also needs to rule out other potentially optimizing agents, such as cultural

evolution and practical reasoning. In the following sections we will first examine Hrdys data to

the effect that the patterns of human maternal investment are adaptive.

2.1.1.Evidence that the FRSi is an adaptation

In order to establish that the FRSi is adaptive (or at least, would have been in the EEA),

Hrdy has to identify the constraints on mothers that would have made a reproductive strategy

that permitted reduced investment or infanticide adaptive. Hrdy does go to some length to

specify what she considers to have been the conditions operating upon women in the

evolutionary past that constrained the sort of reproductive strategy in which they might have

engaged, and how far womens reproductive choices conform to this strategy. The two central

conditions that affect a woman's reproductive choices are the essential limitations on her

fecundity and the inherent costs of each of her infants, requiring, as I described earlier, a pursuit

of quality over quantity in her reproductive choices.

Hrdy specifies what she thinks the conditions are that would result in reduced maternal

investment or infanticide against this background; and also how the woman can be expected to

respond when these conditions are changed. According to Hrdy, the following conditions should

(and do) decrease maternal investment. First, economic hardship and similar stresses often lead

to women reducing investment because a new infant would threaten the viability of the womans


10/32


other offspring or her own reproductive potential (Hrdy 1999: 455, 463). Since a woman has

already invested in her existing children, she needs to protect that investment before taking on a

new child. Second, (apparent) indications of a childs sickliness decrease investment

suggesting it would be a poor investment of her limited resources (ibid.: 453, 455-456, 465-466).

Third, investment often is reduced or ceases when there is close birth spacing, which means the

mother will be trying to stretch her time and energy to care for multiple offspring at once,

threatening her own well being and the welfare of her other offspring (ibid.: 201, 365, 453, 429).

Fourth, reduced investment is often caused by the absence of supportive social networks, in

particular the absence of an investing father, from whom the mother might procure resources or

protection. There are indications that mothers cease investment if they fear that a father will

suspect a child is not his (ibid.: 458-459). Without this social support, mothers are very unlikely

raise demanding human children successfully. Fifth, the youth of the mother is an important

cause of infanticide, since very young mothers rarely have the social networks or knowledge

necessary to raise a child (ibid.:188). The sex of a child can also be a determiner of reduced

investment in cases where sex preferences will lead to greater success for the child or better

treatment for the mother (ibid.: 318-328). Hrdy also argues that the removal of the above

stresses results in a reversion to nurturing behavior (ibid.: 311-315), as does age; the older a

mother is, the more likely she is to take the opportunity to raise a new baby (and treat it very

well) since she may not be presented with any future reproductive opportunities (ibid.: 276, 314-

315).

Hrdy also presents some evidence that human babies may have co-evolutionary

adaptations to deal with maternal ambivalence in the form of increased fatness at birth (as an

advertisement of health) and psychological adaptations to maternal ambivalence in the form of


11/32


patterns of emotional response to improper attachment on the part of the mother (ibid.: 475-484,

516). Although neither of these pieces of evidence is necessarily evidence that maternal

ambivalence itself is an adaptation, they do indicate that maternal ambivalence was a sufficiently

stable problem that babies faced that a selection pressure in favor of such traits was generated.

Finally, Hrdy argues that the FRSi is an adaptation because it exists nearly universally in

the societies in which it could be expected to be adaptive (as she will argue later, societies which

have a history of agrarianism). The idea, presumably, is that the FRSi outcompeted other

strategies especially during certain sorts of evolutionary bottlenecks (ibid: 461-462) and

consequently spread close to fixation. Hrdys evidence for the claim that the FRSi is near

universal takes the form of a widespread pattern of infanticide and child abandonment across

cultures, geographical areas and time periods. She cites evidence from early modern Europe,

modern Europe, south and southeast Asia, Africa and South America to show that mothers

throughout history and throughout the world have been willing to abandon or reduce investment

in babies they were not able or did not want to support. Perhaps Hrdy's most disturbing pieces of

data regard the numbers of infants abandoned at European foundling homes between the

seventeenth and nineteenth centuries. For example, Hrdy quotes numbers for a foundling home

in Milan (343,406 children abandoned between 1659 and 1900,) in Moscow an annual average

of 15,475 infants abandoned, and in Sicily 72,000 infants abandoned between 1783 and 1809.

Overall, many millions of children were abandoned. What is more, the survival rates in these

homes was appalling - in Sicily the survival rate was only 20 percent (ibid.: 297-307).

Obviously one might try to deflect the idea that those abandoning children to these homes were

effectively committing infanticide. The parents might have been hoping the child would do well,

or were ignorant of the conditions to which they committed their children. Hrdy, however,


12/32


suggests that the intentions of the parents were not entirely kindly in committing their children to

such institutions. The appalling survival rate in these homes appears to have been openly

acknowledged (ibid.: 304.) In modern Europe, where infanticide is not openly acknowledged to

be a problem, Hrdy describes the SIDS scandals of the mid 1990's in Britain and the U.S. In

particular Hrdy refers to the case of Waneta Hoyt who lost five children apparently to SIDS. It

turned out that Ms. Hoyt had actually suffocated them. Hoyt was, apparently, suffering from

severe financial and emotional difficulties. The presence of such cross-cultural data is startling,

however, Hrdy has to demonstrate that it is the FRSi that is universal, not simply the

phenomenon of infanticide. Infanticide could occur without being a component of a larger

behavioral strategy. However, I take it Hrdy means that infanticide and reduced investment

occur not just universally but largely in the pattern described earlier in the section, which is

consistent with the existence of an adaptive FRSi.

2.2. Problems for Hrdys hypothesesHrdy does a good case to make that the patterns of infanticide and variable maternal

investment displayed by human mothers are significantly different from those displayed by

primate mothers, and that some of those patterns appear to be adaptive. The central problem for

Hrdys hypotheses is that there are alternative ways to account for these patterns which dont

involve postulating an FRSi adaptation. There are three possible alternative explanations which

could account for some or all of the data: these are psychological disorder, cultural evolution,

and practical reason. The first two are, I think, unsatisfactory for different reasons. However,

the third alternative seems to cause a good deal of difficulty for Hrdy. This is because her

description of the way in which the reproductive strategy functions undermines her claim that


13/32


there is an FRSi that could count as an adaptation in its own right. I will deal with each of the

possibilities in turn and describe why and how far I think they pose a problem for Hrdys two

hypotheses.

2.3.1. Infanticide as a consequence of psychological disorderOne possible explanation for widespread patterns of infanticide that Hrdy describes is

that infanticide is the consequence of a psychological disorder. This would entail that there are

universal human mothering mechanism(s); when they break down, they produce a characteristic

failure to properly nurture infants in the women that have the disorder. If the patterns of

infanticide Hrdy describes are due to this sort of breakdown of proper mothering mechanisms,

then there would not be any FRSi as such (contra hypothesis 1). This suggestion explains Hrdys

data on the universality of infanticide. If it is true that infanticide behavior occurs as the result of

a characteristic breakdown of some common or universal human mental mechanism, then it is

likely to occur with some regularity across time periods and cultures. However, this explanation

does not account for the adaptiveness of the patterns of infanticide if infanticide was a

consequence of damage to proper mothering mechanisms, it is not likely that infanticide would

occur in patterns that would have been adaptive in the EEA.

2.3.2. Infanticide or a complete FRSi as a consequence of cultural transmission

Another possible alternative to Hrdys two hypotheses invokes the power of cultural

transmission and evolution. There are two ways we might employ this notion in responding to

Hrdy. The first is to explain Hrdys data on infanticide behavior as the consequence of social

transmission (in which case there is no FRSi as such, contra Hrdys first hypothesis.) The


14/32


second way is to accept that the behavior women engage in suggests there is an FRSi much as

Hrdy describes, but instead claim the strategy and its adaptive character are the consequence of

cultural evolution. This would suggest that the FRSi is not an adaptation in the ordinary sense,

and that Hrdys second hypothesis was false.

Infanticide behavior as culturally transmitted. On the first view, women acquire a

tendency to engage in infanticide under certain conditions by learning the behavior from others.

The first problem with this explanation of Hrdys data is that infanticide behavior is cross-

cultural and cultural transmission of a behavior is unlikely to occur in all cultures unless there

is some other process involved (such as cultural evolution.) The second problem is the lack of an

obvious means of cultural transmission of infanticide. There is at least some reason to think that

infanticide does persist even where there is very strong cultural resistance to it. Much of Hrdys

data is taken from Early Modern Europe, where the Christian authorities made infanticide both a

sin and a crime. Under these conditions it is unlikely that the people concerned passed on a

tendency to infanticide by social learning since the behavior was undertaken secretly as much as

possible and by individual women or couples. The presence of such institutions as the tour and

the ruota - machines at the walls of foundling homes that allowed an infant to be deposited

safely and anonymously - suggest that the abandonment of a baby was considered sufficiently

shameful that institutions had to go to lengths to get mothers to leave babies somewhere

(relatively) safe (ibid.: 307). Given this sort of cultural environment it seems unlikely that the

massive trend in child abandonment to which these machines were a response was generated by

any form of normal cultural copying2.

The FRSi as the consequence of cultural evolution. The other possibility is that Hrdy is

right that there is a behavioral strategy of the sort that she has described, but that strategy as a


15/32


whole is culturally transmitted via social learning i.e. what is learnt is a conditional strategy

that includes making situation dependent investment in babies. If an FRSi (rather than just

infanticide behavior) is transmitted in this way, it would explain both the universality and

apparent adaptiveness of womens reproductive strategies without it being an adaptation in the

standard sense (contra hypothesis 2)3. Cultural evolution permits selection of a certain sort to

operate on behaviors that are transmitted socially. Familial (vertical) social transmission can

lead to an increase in some behavior B where B improves the genetic fitness of the individuals

that perform it. If an individual has more offspring than others because it performs B, and its

offspring are likely to learn B, then B will tend to spread more rapidly than other behaviors

spread by the same means. For the same reason, sometimes a behavior B can also spread

because groups that contain members that perform B are more likely to grow in size (due to the

fitness increasing effects of being in such a group for the members of that group) than groups

that do not contain B4(for a discussion of the above, see Boyd and Richerson 1985). This

cultural transmission explanation, however, faces some of the problems described in preceding

paragraph cultural transmission of the FRSi would have to includethe social transmission of

conditional infanticide or child abandonment. This component of the strategy would also have to

be socially learnt, while the evidence suggests it was carried out in secret and strongly conflicted

with moral norms that were being socially transmitted at the time.

2.3.2. The FRSi as practical reasonThe third possibility is that the universality and adaptiveness of female reproductive

strategies that Hrdy describes are due to the ordinary workings of maternal practical reason.

Hrdys own suggestion for how the FRSi is supposed to work suggests that what has really


16/32


evolved is a window after birth and before bonding that simply permits the operation of

general purpose practical reasoning mechanism(s) in conditions under which they could not have

operated in primates. This has two consequences. First, it suggests that the adaptive patterns of

behavior Hrdy describes are not due to the operation of an FRSi as such; instead they are simply

the consequence of the operation of another, larger general-purpose practical reasoning

mechanism. Second, it suggests that the pattern of behavior Hrdy has described is not likely to

be an adaptation. One criterion a trait must meet to count as an adaptation is that it must be

mosaic, that is, able to meet Lewontins (1978) quasi independence criterion. In an earlier

(2004) paper, I argued that Lewontins criterion should be read as claiming that a trait T icounts

as meeting the quasi-independence criterion if it can change without causing effects in other

traits T1...Tnwhose total fitness consequences outweigh the fitness consequences of the change

to Ti. If Hrdy is right, then it is unlikely that the reproductive strategy she has described could

be an adaptation.

To be fair to Hrdy, it is quite possible that conditional maternal investment should turn

out to be due to practical reasoning in somesense and at the same time not bejustthe

consequence of a general-purpose practical reasoning mechanism, leaving it able to evolve

independently. There are two main psychological architectures which would have this

consequence. The first type of architecture would involve a general-purpose rational decision

making system - i.e. a psychological mechanism that allows human beings to make rational

behavioral choices given their goals and details about their environmental circumstances but

where the general purpose reasoning system uses representations of FRSi-specific goals and/or

decision rules that would consistently lead a woman to behave in the way described by Hrdy's

FRSi. In other words, the goals and the operational rules would be innate, biological


17/32


representations and the behavior would be produced as a consequence of an interaction between

those innate representations and general purpose rational decision making procedures. So long

as these accompanying innately represented goals are themselves FRSi-specific, then the FRSi

would count as an adaptation in its own right, since changes in the decision rules and

representations specific to the FRSi could permit significant evolutionary change in the

behavioral strategy without ramifying changes and presumably negative fitness consequences -

to other behavioral strategies controlled by the general purpose mechanism. A second

architecture which would allow a rational FRSi to be an adaptation would be one in which the

strategy is generated by an FRSi-specific mechanism that employs decision rules which generate

rational or reasoning-like inferences, and consequently conclusions consistent with appropriate

practical reasoning given the woman's goals and the circumstances in which she finds herself. In

this case, similarly, evolutionary change could take place without negative fitness consequences

elsewhere, since the mechanism would be specific to the FRSi.

However, neither of these views seems to be what Hrdy is suggesting. Where mothers

once attached immediately to their infants, mothers can now employ their ordinary practical

reason to make decisions about whether to keep them or not. Hrdys discussion suggests that the

FRSi simply extends the range of the application of the general purpose goal representations,

decision rules and mechanisms employed in ordinary maternal practical reason, so that they are

employed in a new context. These goals include things like desires to have sufficient food and

other resources, a desire to maintain good social relations with her family and to have a

providing father for herself and her other children, and so on. Presumably these goals and

representations are adaptations they are present because they were selected for and they were

selected for because together with various cognitive mechanisms they reliably generate adaptive


18/32


behavior across a variety of contexts. The problem is, this suggests that while mothers may

engage in adaptive reproductive behavior that does not mean there is an FRSiadaptation as such.

This is because these adaptive representations are presumably employed in other contexts

supposedly not covered by the FRSi. For example, these representations could equally be

involved in how a woman goes about choosing a mate, goes about designing foraging strategies,

and so forth. Suppose that in order to improve the way mothers responded to their infants in

specific contexts, there needed to be a change in the nature of the behavior involved in the FRSi.

Then there would have to be a change in the content of the rules or representations or

mechanisms that are employed in ordinary practical reason. This change would ramify to the

(possibly many) other contexts in which those general-purpose decision rules or representations

are employed. It is likely that these changes would effect how well and efficiently mothers could

reason under these conditions, and reduce the mothers overall fitness. This would strongly

suggest that the FRSi as described would not count as an adaptation in its own right. A much

more likely candidate for an adaptation that permits conditional investment in this case is the

hormonally determined bonding delay the window that occurs in the first seventy-two hours

after birth. This would probably have some fairly direct genetic correlate; and it could likely

evolve relatively independently of the rest of maternal psychology.

3.Hrdys third hypothesis - the FRSi evolved in the last 10,000 yearsPart of Hrdys concern in writing about reproductive strategies in women, is contrasting

that behavior with female reproductive behavior in other primate species. While human mothers

do kill or abandon their own infants under a variety of conditions, primate mothers almost never

do5. In that case we know that conditional maternal investment, if it evolved, evolved in the


19/32


lineage leading to modern humans. The absence of data regarding the reproductive behavior of

early hominids makes it difficult to determine whenin that lineage conditional investment began.

This suggests that the trigger for the evolution of a mechanism that permits conditional maternal

investment (whatever the nature of that adaptation might be) will lie somewhere among the

factors that differ between modern humans and the great apes. Hrdy argues that the FRSi

evolved relatively late in the human lineage that it arose as a response to the changes in birth

spacing that occurred with the onset of the agrarian revolution. As it became easier to wean

babies earlier, women began to have children at much shorter intervals, overloading their

resources. In this section, I mean to take issue with Hrdys position. It is more likely that any

mechanism permitting conditional investment (perhaps the bonding window), if it is an

adaptation, arose due to two main factors. First, the change in the magnitude of the investment

human mothers make relative to primate mothers, and second, the evolution of complex

conditional practical reasoning skills that allowed human mothers to determine when investing in

an infant would not be worthwhile6.

In the following sections, I will first describe the central differences between the

reproductive strategies of women and female primates; second, I will describe which of these

conditions Hrdy thinks are important; third, I will offer some reasons to think that Hrdy is wrong

in her choice of these conditions; and fourth I will present my own view and describe how it

avoids Hrdys problems.

3.1.How Human Mothers are Different from Other Primate MothersHuman mothers share a lot in common with their primate relatives. For one, they are all

mammals, and directly invest a lot of energy in their offspring via lactation after birth. Lactation


20/32


in primates lasts much longer than in any other kind of mammal. Like other primates, humans

also have long gestation periods of many months. Long gestation and lactation makes the

probability of survival of each infant much higher, but it also requires a shift from clutches and

litters of offspring to singleton births. Long birth spacing is vital to all primates, including

humans, in order for mothers to manage the demands of each infant. According to Hrdy, the

consequence of this high level of investment in each infant has meant that, unlike in other

mammals, which selectively invest in some offspring over others, primate mothers have become

almost unconditional in their devotion to their offspring (ibid.: 177-80).

Why then the striking difference in the behavior of female humans? Obviously it is

possible that the behavioral difference is a cultural phenomenon, but as we have already seen it is

not very likely that cultural transmission is responsible for the existence of the complex patterns

of maternal investment Hrdy describes. If these patterns are in part due to the existence of some

sort of adaptation permitting conditional investment, there must be something about the nature of

human evolutionary history that is sufficiently different from primate history to explain them.

There are two main differences between primate and human mothers. The first is that human

mothers make an even more enormous investment in their children than primate mothers do,

even apes. Human gestation is slightly longer than other primates, and labor and delivery much

more dangerous; but these differences are relatively insignificant. What is significant is the fact

that human babies are born much more dependent than primate babies - this is (controversially)

thought be due to the development of a much bigger and more complex brain. Whatever the

reason, human babies require a much longer period of lactation and protection after birth. The

investment of most primate mothers ends at weaning - usually after a few years; the children of

human mothers remain largely dependent until after adolescence - usually well over a decade.


21/32


The second difference between human and primate mothers is that human mothers have

extensive capacities for conditional practical reason, capacities that in their degree are all but

unique to the human lineage. Ape mothers are not capable of figuring out the complex ways in

which a new baby will interfere or assist with their long term reproductive goals7; human

mothers are.

3.2.How the Agrarian Revolution Might have Changed Pressures on MothersHrdy wants to argue that the central reason for the evolution of the female reproductive

strategy to include infanticide is the change in the external pressures on mothers after the

agrarian revolution, rather than early changes in the pattern of infant development and maternal

psychology (ibid.: 105-9, 201-4). The problem faced by human mothers at the agrarian

revolution was the disruption of the carefully balanced primate strategy of distant birth spacing.

The agrarian revolution shortened birth-spacings dramatically. For human mothers this problem

was exacerbated by babies that were all the more expensive than primate babies. Consequently

women began to suffer from having to raise too many children too close together and their

overall reproductive success suffered.

The agrarian revolution changed the lifestyle of many women. Mothers were able to

raise food close to their homes; they had to travel less far each day to gather their food. The food

source, grown under the control of the women themselves, was more reliable than the naturally

occurring food sources they had been using up to that point. Ground grain was also usable as a

weaning food for children. Hrdy points out that a woman's capacity to ovulate depends on the

quantity of fat she has managed to store, the amount of exercise she is undertaking, and how

often she is breastfeeding any babies she might have - sufficiently regular breast feeding


22/32


provides hormonal feedback that prevents ovulation (ibid.: 196). With the agrarian revolution,

women became more consistently well fed due to the reliable food source, and were able to store

more fat; they were also traveling shorter distances to get their food and hence were getting less

exercise. The availability of ground grains as a weaning food encouraged women to get babies

off the breast earlier; thus circumventing the natural birth control provided by lactation.

The consequence was that women were more consistently fertile after the agrarian

revolution than before; and consequently the birth spacing between expensive human infants

began to shorten. This increased the pressures on mothers who had to feed and care for these

expensive babies; and made it more difficult for them to raise each child. Hrdy claims that in

this context, a variation in maternal feelings towards offspring would have been selected for,

since it would allow a mother to decide not to invest in a baby born too soon after another and

hence make the most of her resources.

3.3. Problems for HrdyHaving described Hrdy's argument, I now want to examine several problems with it. The

objections hinge on her claim that it is the change to agrarianism that has resulted in the change

in the female reproductive strategy to include extreme maternal ambivalence and the possibility

of infanticide. There are three central problems with this view.

The first problem is with the distribution of infanticide behavior across societies. Hrdy's

claim is that it was the change to agrarianism in human societies that provoked the evolution of

the FRSi. This should mean that, barring extensive intermarriage and gene flow between

societies, infanticide should be much more common in societies that have changed to

agrarianism since the Pleistocene than in societies which have never been agrarian. This should


23/32


mean that many hunter-gatherer societies, such as the !Kung San and the Ache should not show

much evidence of infanticide. Nevertheless, this is not the case, as Hrdy herself describes

elsewhere in the book. !Kung mothers routinely dispose of babies that are born ill, deformed or

disabled - a mother goes out into the bush, and if she does not return with a baby, no-one

questions it. Similarly, the Ache seem to not only accept infanticide, but to encourage it under

certain circumstances (ibid.: 453). There is also a rather nice case Hrdy describes in which a

group of Bantu (agrarian) mothers convince a !Kung mother not to abandon a child - Hrdy's

point being that amongst the Bantu children are more highly valued than among the !Kung (ibid.:

374-5). Hrdy claims that infanticide is not as common amongst hunter gatherers as it is in non-

agrarian societies, and hence we need an explanation for why there is more infanticide in

agrarian societies (ibid.: 184). But, this does not explain her data regarding the Bantu, nor, given

that the primate model is an almost complete absence of maternal infanticide, the presence of any

significant amount of infanticide amongst hunter gatherers. Even Hrdy admits this is because the

natural safeguards against close birth-spacing and other reproductive pressures on mothers arise

less often amongst hunter-gatherers. This suggests that women in both sorts of societies have the

FRSi, but due to lower reproductive pressures, the FRSi leads to less actual infanticide behavior

amongst hunter gatherer women.

The second problem for Hrdys view is that agrarianism was not all bad for mothers.

Agrarianism also makes life somewhat easier for women, allowing them to grow patches of food

nearer their homes, reducing travel time and making that food source more reliable. The reason

birth spacing becomes shorter is that mothers are more quickly able to wean children - because

there is a more ready supply of food to hand. With more reliable food sources, women are

healthier and better able to support and care for the infants that come along. They also spend


24/32


less time away from their villages and more time in places where there are other people and

family members to help with the children (ibid.: 374-6). Furthermore, as I have just mentioned,

some agrarian societies seem better disposed to children regardless of quality, than hunter

gatherer societies. In other words, it seems that although agrarianism made some of the burdens

on mothers heavier, it also should have made some contributions to their relative well being.

Consequently, it is not clear whether agrarianism overall was better or worse for mothers, nor

whether agrarianism was so much worse that there was a very rapid evolution of conditional

maternal investment. Of course, agrarian food production is not by any means 100% reliable;

Hrdy might argue that since infanticide is supposed to be conditional, maybe the vagaries of

climate and so forth on food production, with one good year shortening a birth spacing, followed

by years of hardship, were what was really responsible for the evolution of an FRSi.

The third problem with suggesting that it was the agrarian revolution that provoked the

evolution of the FRSi is that it requires a very short period of evolution - less than 10,000 years.

Is it really possible that such a short span of time could produce a behavioral adaptation of the

kind that Hrdy suggests? Hrdy claims that it is, and cites another example of what she believes

is an obvious adaptation to the circumstances of the agrarian revolution - the appearance of

lactose tolerance among people dependent on herding livestock. Babies - in fact all mammalian

young - are born with the ability to digest lactose, but in adults this ability usually vanishes.

Adults that drink milk are unable to make use of the sugars that are in it. However, some human

populations, notably most of the Europeans and many of the herding populations in Africa such

as the Bantu and the Tutsi retain the ability to digest lactose into adulthood. Human populations

that do not have a history of relying on herding rarely have the ability to digest lactose (ibid.:

108). The idea is that the lactose tolerance evolved after the agrarian revolution in which


25/32


26/32


difference between them and humans seems to be quantitative rather than qualitative. It is

possible that the massive increase in the dependency of human infants crossed some sort of

threshold value that made a worthwhile strategy of avoiding commitment to a baby one couldnt

manage or that would be a waste of an enormous reproductive effort. In addition, once human

beings had developed sufficiently complex conditional practical reason, it became possible for

mothers to predict how well babies with certain properties would do, and to weigh up the relative

likelihood of danger to herself and her other children. However, the use of practical reason in

regard to their infants would not have been possible for mothers who had bonded to their

children in old primate sense. What was needed was a change in the bonding patterns that

mothers had with their infants. Mothers who did not bond immediately had an advantage over

mothers that did.

The point at which conditional maternal investment via the bonding window became

possible for human mothers seems most likely to have occurred with the origin of the massively

lengthened dependency of human infants, and the acquisition of accurate long term conditional

reasoning skills. The change in the length of childhoods happened fairly late in hominid history

(later than the australopithecines (Conroy and Kuykendall 1995), just beginning to emerge with

the evolution ofHomo erectus(Smith 1994)). It is less clear exactly when accurate conditional

practical reasoning might have evolved. However, both events would have likely occurred well

before the agrarian revolution.

This explanation improves on Hrdys in two ways. The first way is that it better accounts

for the distribution of infanticide behavior across societies. If conditional maternal investment

arose early in hominid history, then it arose before the origin of any particular human society,

explaining why this sort of maternal behavior occurs in all societies. The second way is that it


27/32


associates the change in human strategies with a more significant change in the nature of

maternal reproductive investment and maternal psychology than merely the alteration in the

environmental circumstances mothers faced. After all, female reproductive decision making is

supposed to be able to deal with conditional changes in environmental circumstances. The

agrarian revolution simply extended the challenges that already faced human mothers

occasionally close birth spacing, famine, and so forth - it just made those challenges occur

more often. Hrdy suggests that the FRSi is conditional hence it would be adaptive regardless

of how often the conditions necessary to produce infanticide as sucharise (as long as such

conditions are not very rare). Hence it is unlikely to be a mere increase in the occurrence of

those conditions that provoked the evolution of the FRSi.

4. ConclusionTo conclude, I think that Hrdy has difficulty establishing any of her three hypotheses.

Hrdy's main problem with her first two hypotheses is in ruling out the possibility that conditional

maternal investment is simply the consequence of ordinary practical reasoning rather than an

FRSi. Her main suggestion for a psychological architecture for the FRSi seems to support this

idea; that what is really the adaptation is a hormonal window that gives mothers a period of

time in which they do not readily bond with their children. Mothers can then rationally decide

how much to invest in their infant given their other desires and goals.

Hrdy does have some evidence that the patterns of maternal investment she is describing

would likely be adaptive under the conditions of the EEA, and women do seem to behave

according to those patterns. However, Hrdy suggests that the patterns of conditional investment

she describes are generated by the application of ordinary practical reason in a new context. If


28/32


such patterns of investment are produced by the application of ordinary practical reason, then it

is likely that the FRSi violates the quasi-independence criterion for being an adaptation.

The third hypothesis is the most problematic, that the FRSi evolved in the last 10,000

years. First, it is not clear that a complex behavioral adaptation could arise in this very short

period of time. Second, it is not clear that the agrarian revolution was entirely bad for mothers -

at least not so much that it would have provoked the evolution of an FRSi. Third, women who

belong to societies which did not go through an agrarian revolution still show signs of

infanticide. It seems much more likely that conditional maternal investment (perhaps in the form

the window that Hrdy suggests), if it is an adaptation, is an adaptation to the change in the size

of the investment that a human mother must make in her child from the original primate model,

permitted by increases in the human capacity for long-term conditional practical reasoning.

Human mothers, faced with an even more extreme investment than their primate relatives

together with the potential to evaluate whether such an investment was worthwhile, became

capable of delaying and ceasing investment in infants that would have been beyond their

capacity to raise successfully.


29/32


30/32


own infants.

6Many thanks to an anonymous reviewer for pointing out the potential importance of this

particular difference for Hrdys hypothesis.

7There is some debate about the extent of the conditional and causal reasoning abilities primates

possess. There is some evidence that even monkeys can make good inferences about how other

animals and objects will behave (e.g. Hauser et al. 2002) but some primatologists are skeptical

about how far such apparent abilities go (e.g. Tomasello 1999). It is unlikely that primate

mothers possess the sort of long term reasoning skills to make the sort of conditional judgments

necessary to accurately determine the contribution of a new infant to their overall reproductive

success.

8Also note that this adaptation may have been generated by an exceptionally strong selection

pressure due to gene-culture co-evolution via a cultural environment in which milk and milk

products were available, plus large scale migration to high latitudes where UVB radiation was

low and hence the ability of humans to absorb calcium via vitamin D was reduced. Lactase

production permits both the digestion of milk sugars, and also the ingestion of vitamin D (see

Durham, 1991 for an excellent discussion of this hypothesis).


31/32


ACKNOWLEDGEMENTS

I would like to thank Stephen Stich, Karen Neander, Brian Loar and Tim Maudlin, the members

of the philosophy department at Dartmouth College and several anonymous reviewers for their

very helpful comments on earlier versions of this paper.


32/32


REFERENCES

Boyd, R. and Richerson, P.J.: 1985, Culture and the Evolutionary Process, The University of

Chicago Press, Chicago.

Conroy, G. C. and Kuykendall K.: 1995, Paeleopediatrics; or when did human infants really

become human?American Journal of Physical Anthropology, 98, 121-131.

Driscoll, C.: 2004, Can Behaviors be Adaptations? Philosophy of Science, 71, 16-36.

Durham, W.: 1991, Coevolution: Genes, Culture and Human Diversity, Stanford University

press, Stanford.

Fuchs, R. G.: 1987, Legislation, poverty and child abandonment in nineteenth-century Paris,

Journal of Interdisciplinary History, 18, 55-80.

Gagneux, P., Woodruff, D. S., and Boesch, C.: 1997, Furtive mating in female chimpanzees,

Nature, 387, 358-359.

Hauser, M.D., Pearson, H. & Seelig, D.: 2002, Ontogeny of tool use in cotton-top tamarins

(Saguinus oedipus): Innate recognition of functionally relevant features,Animal Behaviour, 64:

299-311.

Hrdy, S. B.: 1999,Mother Nature: A History of Mothers, Infants and Natural Selection,

Pantheon/Random House, New York.

Lewontin, R. C.: 1978, Adaptation, Scientific American,239, 212-230.

Smith, B. H.: 1994, Patterns of dental development inHomo, Australopithecus, Pan and

Gorilla,American journal of Physical anthropology,94, 307-325.

Tomasello, M.: 1999, The Cultural origins of Human Cognition, Harvard University Press,

Cambridge, Massachusetts.

killing babies hrdy on the evolution of infanticide

Documents