Leda Cosmides

Introduction

Leda Cosmides (born 1957) is an American psychologist who, with her husband and collaborator John Tooby, did more than anyone else to define evolutionary psychology as a research programme. Trained first as a biologist and then as a cognitive psychologist, she argued that the human mind is not a general-purpose reasoning device but a collection of specialised mechanisms shaped by natural selection to solve the recurring adaptive problems of ancestral life. Her most consequential single result was experimental: people who fail a standard abstract logic task perform well on the very same problem when it is framed as detecting someone who has cheated on a social exchange.

That finding, and the research programme built around it, made Cosmides a central figure in late twentieth-century psychology and, equally, a target. Her work sits at the centre of the argument about how much of human behaviour can be explained by ancestral adaptation, and critics have challenged both her specific interpretation of the reasoning data and the broader assumptions of massive mental modularity.

Early Life and Education

Cosmides was born in Philadelphia into a Greek-American family; her parents, George Cosmides and Nasia Cosmides, helped found a Greek Orthodox church in Bethesda, Maryland. She studied biology as an undergraduate at Radcliffe College and Harvard University, taking her bachelor's degree in 1979, and while there came under the influence of the evolutionary biologist Robert Trivers, whose work on reciprocal altruism, parental investment and parent-offspring conflict supplied the theoretical scaffolding she would later bring to psychology.

She then moved into cognitive psychology, completing a doctorate at Harvard in 1985. Her doctoral research asked whether a Darwinian account could explain a famous anomaly in the reasoning literature: people perform badly on Wason's abstract selection task but much better when the same logical structure is presented as a rule about social exchange. That question produced her 1989 article in Cognition, one of the most cited papers in the reasoning literature, and it established the pattern of her career, in which an evolutionary hypothesis is tested against a well-documented psychological phenomenon rather than merely asserted.

Academic and Professional Career

After her doctorate Cosmides did postdoctoral work with Roger Shepard at Stanford University. In 1991 she joined the faculty of the University of California, Santa Barbara, where she became a full professor in 2000 and where she and Tooby co-founded and co-direct the Center for Evolutionary Psychology.

Her work in the 1990s moved between three registers. She continued the experimental programme on reasoning, publishing tests of the social contract hypothesis with collaborators including Gerd Gigerenzer and, later, cross-cultural work with the Shiwiar of Amazonian Ecuador. She wrote the field's canonical statement of purpose, including the introductory chapter of The Adapted Mind and, with Tooby, a series of theoretical papers laying out the logic of adaptation, the concept of the environment of evolutionary adaptedness, and the argument that the mind is composed of many domain-specific programmes rather than a few general ones. And she helped build the institutional infrastructure of a new field, editing volumes, training students and arguing publicly with critics.

Her work has been recognised with the American Association for the Advancement of Science Prize for Behavioral Science Research in 1988, the American Psychological Association Distinguished Scientific Award for an Early Career Contribution to Psychology in 1993, a Guggenheim Fellowship, the National Institutes of Health Director's Pioneer Award in 2005, and the Jean Nicod Prize in 2020. She was elected to the American Academy of Arts and Sciences in 2023.

Major Contributions to Psychology

Cosmides' central empirical claim is that natural selection equipped humans with cognitive machinery specialised for social exchange, and specifically for detecting cheaters, that is, individuals who accept a benefit without paying its cost. On the standard Wason selection task, most people fail to check the cases that would falsify a conditional rule. Reframe the identical logic as a rule of the form "if you accept a benefit, you must satisfy its requirement," and performance improves dramatically. Cosmides argued that this content effect is not a matter of familiarity or of general deontic reasoning, because the improvement is specific to situations in which cheating is possible and to the perspective of the party who might be cheated, and because it disappears when the rule is reframed so that cheating is impossible.

Massive modularity and the adapted mind

With Tooby she developed the broader architecture within which that result is interpreted. Their argument holds that a general-purpose learning device could not have solved the problems ancestral humans faced, because such problems are too varied and underdetermined for unguided learning, so that the mind must contain a large number of content-specialised programmes, each with its own procedures for the domain it governs. The 1992 volume The Adapted Mind, edited with Tooby and the anthropologist Jerome Barkow, assembled work across domains, from mate choice to coalitional reasoning to spatial cognition, and became the field's founding text.

A third contribution was polemical. Cosmides and Tooby argued that mainstream social science rests on an unstated and mistaken assumption that the mind is a general-purpose, content-free device shaped almost entirely by culture, an assumption they labelled the standard social science model. Much of their theoretical writing is devoted to showing that this picture is not merely incomplete but incompatible with what is known about evolution and about the nervous systems of other animals.

Major Publications

Cosmides, L., & Tooby, J. (1981). Cytoplasmic Inheritance and Intragenomic Conflict. Journal of Theoretical Biology, 89.
Cosmides, L. (1989). The Logic of Social Exchange: Has Natural Selection Shaped How Humans Reason? Cognition, 31.
Barkow, J. H., Cosmides, L., & Tooby, J. (Eds.) (1992). The Adapted Mind: Evolutionary Psychology and the Generation of Culture. Oxford University Press.
Cosmides, L., & Tooby, J. (1992). Cognitive Adaptations for Social Exchange. In The Adapted Mind. Oxford University Press.
Tooby, J., & Cosmides, L. (Eds.) (2000). Evolutionary Psychology: Foundational Papers. MIT Press.
Tooby, J., & Cosmides, L. (2005). Evolutionary Psychology: Conceptual Foundations. In D. M. Buss (Ed.), Handbook of Evolutionary Psychology.

Impact and Legacy

Whatever one thinks of the conclusions, Cosmides changed the terms of debate. Reasoning research after her work had to explain content effects rather than ignore them; the study of cooperation, punishment and fairness acquired an evolutionary vocabulary; and a generation of researchers in psychology, anthropology and economics adopted the practice of asking what function a psychological mechanism might have served. The Center for Evolutionary Psychology at Santa Barbara trained many of them.

The criticisms fall into two groups. The first concerns her specific interpretation of the reasoning data. At nearly the same time as Cosmides developed the social contract account, Patricia Cheng and Keith Holyoak proposed that people reason about realistic situations using pragmatic reasoning schemas, chiefly deontic schemas of permission and obligation, acquired through ordinary experience rather than prepared by selection. In a 1989 exchange they argued that Cosmides' results could be explained without positing an evolved cheater detection module. Gerd Gigerenzer and Klaus Hug replicated her findings in 1992 and concluded that the crucial variable is pragmatic rather than semantic, namely whether a person is cued into the perspective of a party who can be cheated, a conclusion they took to support social contract theory and to defeat the schema account. Other researchers have gone further in the deflationary direction: Almor and Sloman argued that performance on the selection task is explained by ordinary knowledge operating during text comprehension, so that no specialised mechanism is needed at all, and Cummins proposed that deontic reasoning reflects an older primate adaptation for managing risk in dominance hierarchies rather than a social exchange module. Taken together, this literature establishes that the facilitation effect is real and robust while leaving its explanation genuinely contested.

Cosmides and Tooby's response has been to specify predictions that distinguish the accounts. They argued that the social exchange algorithm predicts patterns that deontic schemas do not, in particular that performance tracks the possibility of cheating rather than the presence of obligation, and that switching the perspective of the parties in a bilateral contract flips which cards people choose. Work with the Shiwiar, a small-scale Amazonian society, was presented as showing that the phenomenon is not an artefact of Western schooling, and Tooby and Cosmides have answered the modularity critique by insisting that domain-specificity is a prediction of any serious computational account of the mind, not an optional extra.

The second group of criticisms targets the wider programme and is harder to settle. Critics argue that adaptationist explanations are difficult to falsify, since almost any trait can be given a plausible ancestral function after the fact, a worry sharply stated in evolutionary biology by Stephen Jay Gould and Richard Lewontin and pressed against evolutionary psychology by philosophers including David Buller. They object that the environment of evolutionary adaptedness is too poorly specified to generate precise predictions, that the standard social science model is a caricature of what social scientists actually believe, and that the massive modularity hypothesis has not delivered the catalogue of identified mechanisms it promises. Defenders reply that the same charge of unfalsifiability could be made against any historical science, that evolutionary psychology generates testable and sometimes surprising predictions, and that the alternative research programmes have explained less. That argument remains open, and Cosmides' work is where most of it is conducted.

Learn More

John Tooby
David Buss
Charles Darwin
Steven Pinker
E O Wilson — whose sociobiology set the evolutionary frame that evolutionary psychology later worked within

References

Cosmides, L., & Tooby, J. (1992). Cognitive adaptations for social exchange. In J. H. Barkow, L. Cosmides, & J. Tooby (Eds.), The Adapted Mind (pp. 163-228). Oxford University Press.