evidence and causation in biology and economics michael joffe imperial college london canterbury,...

Evidence and causation in biology and economics

Michael Joffe  Imperial College London

Canterbury, September 2012

Structure of the presentation • a concept of causation – epidemiology – physiology

• systems in biology – physiological systems – population biology – evolutionary biology

• systems in economic theory • conclusion

Structure of the presentation • a concept of causation – epidemiology – physiology

• systems in biology – physiological systems – population biology – evolutionary biology

• systems in economic theory • conclusion

A concept of causation I

• a causal relationship is one that has a mechanism that by its operation makes a difference 

• difference-making: a change in the probability and/or timing of an event, or in its magnitude or severity – includes partial/multiple, stochastic and deterministic

causation, plus counterfactual and manipulationist accounts

– but excludes chains, webs, cycles – single link only • this is compatible with classic accounts of

causation in epidemiology (Bradford Hill, Rose)

A concept of causation II • not all differences correspond directly to a causal

relationship – something has to make a difference, over time – no direct mechanism responsible for the sex difference

in breast cancer incidence – it is due to metabolic difference between the sexes – these do play a causal role over time, i.e. upstream causes

• mechanism and difference-making are properties of the causal relation, and seen as complementary – discovery of either can come first – a totally convincing explanation includes both

A concept of causation III • mechanism: powers/capacities [Cartwright; MDC]• combining mechanism and difference-making is

similar to the Russo/Williamson epistemic theory but takes an ontic view: asking what is the source of evidence in the real world – science can uncover evidence of the structure (“what”)

and mode of operation (“how”) of a mechanism – and of the difference it makes (“that”) – quantitative

or qualitative

• asymmetry requires an ontic perspective: beliefs do not alter reality, but reality can alter beliefs

“Physiology” • takes a primarily mechanistic view – how the

body works – but typically starts from an observed difference (often qualitative): – dietary protein is broken down – how? → pepsin, its

structure, etc – not necessarily complicated – a nerve impulse crosses a synapse – how?

• the mechanistic evidence is then juxtaposed with more difference-making evidence, e.g. many-one synapses and their quantitative characteristics

• physiological systems are evolved => regularity; the difference they make controls their evolution

Epidemiology • a difference-making approach: demonstrating

that the rate of a disease differs in groups defined by their exposure

• causal inference: the demonstrated difference is not due to e.g. chance, confounding or selection – mechanisms are often sought e.g. biomarkers

• evidence of mechanism is complementary: how the damage occurs

• any postulated cause must be plausible (yellow fingers); in due course the complementary biochemical pathways need to be elucidated

Structure of the presentation • a concept of causation – epidemiology – physiology

• systems in biology – physiological systems – population biology – evolutionary biology

• systems in economic theory • conclusion

Physiological systems I • until now, we have focused on a single link • organisms are composed of chains, webs and

cycles of causal links – each being a mechanism that makes a difference

• the dominant feature of physiological systems is homeostasis: a ± constant internal environment – core body temperature – numerous chemical concentrations

• the system property is constancy – a difference-making property

Physiological systems II • how does the causal concept of mechanism and

difference making fit with a system of this kind? • body temperature maintenance (homeothermy) – shivering is muscle activity (mechanism) that raises

temperature (the difference) – sweating is fluid secretion (mechanism) that lowers

the temperature (the difference)

• so: each link has mechanism + difference-making • the system only has its own difference-making

properties: relative constancy due to balancing or compensating (negative) feedback

Balancing fb response to external “shock”

Physiological systems III • in general, a system of this kind has numerous

links, each a mechanism that makes a difference, as well as a characteristic mode of operation – the difference-making of the system as a whole

• this can be regarded as system or endogenous causation – the system is relatively insensitive to initial conditions [Forrester 1970; Lane 2007]

• a focus on individual links is “reductionist”, in contrast to system “emergent” properties

• evidence is obtainable for all three categories

Other biological systems I • in population biology/ecology, population growth

is exponential (reinforcing, or positive, feedback) • carrying capacity: logistic growth (–ve fb too) • Lotka-Volterra classic predator-prey model: – predation and reproduction are its component links,

each with mechanistic and difference-making aspects – the system property is that the population sizes

fluctuate systematically – a property of observed ecosystems and of the systems model

– this pattern results from balancing feedback with delay, a classic pattern in system dynamics

Some biological systems

Other biological systems II • in evolutionary biology, a different pattern is

frequently observed: – the growth propensity of each tree threatens to

obscure the sunlight of other trees, leading them to compete over evolutionary time – they all grow tall

– a plant evolves the capacity to poison animals that eat it → some of the animals develop the ability to deal with the toxin, likely → further response by the plant

• this is an arms race, a form of reinforcing (positive) feedback, tending to produce exponential growth – although this can be limited by carrying capacity

Summary of biological systems • like all causal systems, they are composed of links,

each being a mechanism that makes a difference • to be called a system (in this sense), they have to

possess an additional “emergent” property of system or endogenous causation – a difference-making characteristic that results from the way that the links combine

• this system property is just as real in its effects as its component links – even in systems that are not organised (outside physiology) – not “just a model”

• systems can also be subject to exogenous causes

Structure of the presentation • a concept of causation – epidemiology – physiology

• systems in biology – physiological systems – population biology – evolutionary biology

• systems in economic theory • conclusion

Economic systems I • the fundamental topic of study in economics is

“the” market • system property of convergence towards a stable

equilibrium has been recognised since Adam Smith

The standard market equilibrium model

price

quantity

P1

Q1

D

S

Economic systems I • the fundamental topic of study in economics is

“the” market • system property of convergence towards a stable

equilibrium has been recognised since Adam Smith

Economic systems I • the fundamental topic of study in economics is

“the” market • system property of convergence towards a stable

equilibrium has been recognised since Adam Smith • this is a typical simple balancing feedback system,

like those of homeostasis – but in a system that is not evolved or deliberately organised – hence it is an idealisation or abstraction from real economic life – it could still capture the underlying essence though – does it? for all types of market?

price A quantity

demanded A-

intensity of competition

quantity supplied A

cost A

--

//profit/incentive

price A quantity

demanded A-

intensity of competition

quantity supplied A

cost A

--

//profit/incentive

A SYSTEM WITH COMPENSATING (NEGATIVE) FEEDBACK – IT TENDS TO MOVE TOWARDS STABLE EQUILIBRIUM

price A quantity

demanded A-

intensity of competition

quantity supplied A

cost A

--

//profit/incentive

a “supply shock” – e.g. a better

harvest than usual

price A quantity

demanded A-

intensity of competition

quantity supplied A

cost A

--

//profit/incentive

a “demand shock” – e.g. a successful

promotion campaign

price A quantity

demanded A-

intensity of competition

quantity supplied A

cost A

--

//profit/incentive

THESE PROCESSES OCCUR OVER TIME, BUT ARE STATIC IN THE SENSE THAT THE ONLY TIME-

DEPENDENT ENDOGENOUS PROCESS IS TOWARDS A STABLE EQUILIBRIUM

a “supply shock” – e.g. a better

harvest than usual

a “demand shock” – e.g. a successful

promotion campaign

Some economic series

Economic systems II • balancing feedback with delay: e.g. construction

cycles in property markets; business cycles? • bubbles: trend extrapolation that causes a self-

fulfilling prophecy – reinforcing feedback • financial markets: bubbles-prone, highly volatile,

and with some endogenous chaotic properties • capitalist growth: firms’ control over the means

of production gives them flexibility over costs and the size of the market they can supply – an arms race – reinforcing feedback

Economic systems III • balancing feedback is a feature of economic

systems, but is frequently joined by other types of feedback, giving the system different properties

• neither the conventional view, that markets are always self-correcting, nor the critical view, that they do not have this property at all, is correct

• this is not the same issue as “market failure” • fluctuations may occur, due to – ve fb with delay• to understand bubbles, or capitalist growth, it is

necessary to understand reinforcing feedback

Economic systems IV • the properties of the system primarily depend on

its feedback structure • rationality vs. realistic behaviour is a secondary

issue – all that is needed to make the system work is some degree of regularity, especially in response to incentives – situational rationality

• rationality is useful for mathematical modelling• many economists see theory/models not based

on optimisation/strict rationality as “ad hoc” – this is a basic category error

Structure of the presentation • a concept of causation – epidemiology – physiology

• systems in biology – physiological systems – population biology – evolutionary biology

• systems in economic theory • conclusion

Conclusion • there are cross-cutting methods of analysis that

are applicable across many disciplines • system dynamics – cyclical combinations of

causes containing feedback loops – is one; another is complex systems (complexity, chaos)

• systems with feedback have characteristic modes of behaviour: endogenous causation, a difference-making property; their constituent links have mechanism + difference-making

• different market types have radically different properties, resulting from feedback structure

THANK YOU!