top of page
  • Writer's pictureDale DeBakcsy

Beyond Nature Vs. Nurture: Marian Cleeves Diamond and Leda Cosmides

Updated: May 9

In 1964, two publications announced the beginning of two roads out of the centuries-long quagmire represented by the Nature Versus Nurture debate among philosophers, psychologists, and social theorists. The first, “The Effects of an Enriched Environment on the Histology of the Rat Cerebral Cortex,” was the opening move in a multi-decade program by Marian Cleeves Diamond (1926-2017) to demonstrate that it’s not a question of innate vs learned, but rather of how the two interface, with environment producing measurable changes in the baseline physical characteristics of the brain. The other, “The Genetical Evolution of Social Behavior” by W.D. Hamilton, started the ball of modern evolutionary psychology rolling by positing how different degrees of altruism could be explained as a mathematical maximization of gene survival whereby individuals might give up their right to reproduce and even their very lives if it served the purpose of ensuring their kin’s overall genetic survival.

The analysis of social behaviors through the lens of evolutionary theory would become the hallmark of Leda Cosmides (b. 1957), who with John Tooby would edit and contribute to the classic text The Adapted Mind (1992), which sought once and for all to bury any remaining vestiges of the limiting Nature vs. Nurture debate, replacing them with a larger interest in how our common neural architecture came to be that places the focus on our species-wide commonalities instead of on questions of the micro-tuning of our common features through individual experience and environment. Taking it as given that research like Diamond’s showed what it claimed to show, and that environment can play a role in nudging basic architecture a bit this way or that way, Cosmides then pushed to the much wider problem of how that architecture came to be, and whether it is based on general structures that we adapt to particular situations, or a myriad of finely tuned neural circuits each of which has been calibrated over 10 million years to deal with one very particular situation.

Marian Cleeves was born in Glendale, California, in 1926. Her father was a British doctor who built something of an idyllic paradise on 20 acres of land, constructing playsets and tennis courts for the children to romp on when they weren’t sampling liberally from the fruit trees or tending to the chickens. Her mother was also highly educated, having studied German literature, Greek, and Latin at the University of California, Berkeley (or Cal), an institution which would play a central role in her daughter’s academic life. Cleeves knew she wanted to go to Cal, but decided to attend Glendale Community College first to extend the amount of time she could spend with her family. She attended Berkeley starting with her junior year, graduating in 1948, and enrolling straightaway in the graduate program there, earning her Master’s in 1949 for research she did on referred pain (a phenomenon whereby you feel pain in a location other than that where an injury is sustained).

While in graduate school, Cleeves lived at the International House (which is still there today), where she met an athletic young aspiring physicist by the porn-ready name of Dick Diamond, whom she married in 1950, and subsequently followed to Harvard in spite of not having an academic position lined up for herself either there or anywhere nearby. She received her PhD in 1953 the same month she had her first of four children, and eventually found her way to a part time research project at Cornell with Marcus Singer that turned into a three year instructor’s position when she was hired at the last minute to fill in for Singer, whom the university sacked for refusing to divulge the names of other professors with whom he had attended a Communist Party information meeting.

While teaching at Cornell, one day she happened to read in Science magazine about an experiment that had been performed at Berkeley by David Krech, Edward Bennett, and Mark Rosenzweig which had demonstrated that a rat strain that had been bred for hyper-competence at running mazes showed markedly higher acetylcholine levels in their brains than those who had been bred for their hyper-incompetence at running them. It was a tantalizing result that suggested that learning ability had a chemical basis, and in her autobiographical sketch, Diamond reported immediately being struck by the possibility that not only the chemicals of the brain, but the very anatomy of the brain itself, might change in response to learning. She would get the chance to put that theory to the test when, in 1959, as a result of her husband’s acceptance of a job at Cal, she was in a position to approach Krech, Bennett, and Rosenzweig directly with her research proposal.

Her experiments compared the brain anatomy of rats who had been brought up from their 25th to 105th days of life in a stimulation rich environment full of toys and other rats as against that of rats who had been raised in a solitary environment with access to just food and water, and in enriched rat after enriched rat she found a consistent 6% increase in the thickness of the cortex, a bombshell of a result which showed learning and enrichment having a direct, measurable, and physical impact on the properties of the brain itself. She published those results in the 1964 paper we mentioned at the beginning of this piece, and to dispel criticism that what she had observed was just the hastening of the pace of brain maturation, she spent seven years laying out the groundwork for the baseline cortical development of male and female rats against which all future results could be compared.

One of the most significant results to come out of this baseline work was the establishment of gender distinctions in brain structure, with male rats featuring a thicker right cortex, which in old age settled down to a greater symmetry between the hemispheres, while female rats tended towards hemispherical symmetry their whole lives. Follow-up experiments in the 1960s and 1970s on the neural impact of enrichment found not just an overall thickening of the cortex in enriched rats, but greater dendritic branching, higher glia to neuron ratios, larger synaptic junctions, and perhaps most comfortingly for us, that even elderly rats, when exposed to enriched environments, were capable of thickening their cortices some 10% more than their unenriched brethren, a most welcome result suggesting that some capacity for neural plasticity remains into old age.

In the 1980s, Diamond divorced her husband in order to marry Professor Arne Scheibel, an emotionally expressive widower with a great and open curiosity for the world, and embarked on a new direction in her research, tracing the possible connections between neural activity and the stimulation or inhibition of the immune system. At the same time, she co-authored the massively popular Human Brain Coloring Book (1985) and wrote up the results of her three decades of research on the neural impact of enrichment for a popular audience in Enriching Heredity: The Impact of the Environment on the Anatomy of the Brain (1988).

As Diamond was winding down her incredibly impactful career, just across the San Francisco Bay at Berkeley’s great rival, Stanford, Leda Cosmides was wrapping up her postdoctoral work and was busily constructing the foundation of a new approach to evolutionary psychology which would not shy away from the deep and treacherous waters of the evolutionarily derived neural circuitry at the basis of social transactions. By 1985, when Cosmides published her dissertation, Deduction or Darwinian Algorithms? An Explanation of the “Elusive” Content Effect on the Wason Selection Task, the intellectual descendants of W.D. Hamilton had already advanced the idea of evolutionary psychology from the realm of insect altruism into that of human behavior, with Donald Symons’s 1979 The Evolution of Human Sexuality representing a particular milestone in the revitalization of the field. Evolutionary psychologists (EPs) believed strongly that the diverse capacities of the brain developed as adaptations which solved particular problems of survival deep in humanity’s past, and that the human mind, then, is the result of the interlinked running of highly specialized sub-modules, rather than of the specialization through experience of a general brain that comes pre-loaded with only a few rational or logical functions that it applies to different situations as they arise.

In one of Cosmides’s memorable examples, she points out that humans need carbohydrates to survive, and dung beetles need poop, and that as a result one species developed an instinctive circuit that responds positively to sugar but resolutely not to poop, and the other a circuit that responds positively to poop and, well, likes sugar all right but wouldn’t build a house for the kids out of it. Ancestors to humans might at one point have had a circuit that drove them to find poop delicious, but they likely died out very quickly, leaving behind a group of pre-humans with all more or less the same mental mechanism that responds to the taste of sugar with a, “Yes, Good, Let’s Eat More of That Please,” which has allowed us to consume the calories we have needed to survive as a species.

Studies of infants in particular lend credence to the ideas of the evolutionary psychologists. Instead of the blank slates of the Empiricist tradition, babies come pre-wired with a number of astonishing abilities and ideas, the result of basic neural modules that proved useful throughout humanity’s long past, including instinctive attraction to faces, a basic sense of causality, a distinction between animate and inanimate objects, and ideas about objects as solid and unitary. Humans are not beyond animal instincts from an EP perspective, they have just made higher order intellectual categories instinctual through the development of specialized neural circuits.

Cosmides extended these ideas into the realm of social interactions through a famous adaptation of the Wason selection task, which presents a logical statement and then a series of cards related to that statement. For the statement, for example, I could say “If you cast Fireball, then you are a Fire Mage.” I would then present you with four cards, which have a spell on one side and a fantasy class on the other, and only show you one side of each, like [FIREBALL], [FIRE MAGE], [ICEBOLT], [LIGHTNING MAGE], and ask you which ones you have to turn over in order to discover if the rule that I gave you was violated.

People are not very good at this, as a rule, with only something on the order of one out of every four people doing it correctly (by the way, in this example, the two you would need to flip are FIREBALL and LIGHTNING MAGE). Spotting situations where violations might occur requires logical thought that it can be difficult to work our brains around to, particularly if we haven’t taken a logic or geometry class in a while, but Cosmides believed that, since humans must come with a whole battery of specialized social circuits in order to navigate the complicated web of expectations of social existence without getting cheated, people might perform distinctly better on the Wason task if the statements and the resultant cards had a social exchange dimension. If instead of talking about mages and fireballs, we say, “If you watch the baby, then you get $20” and present cards which have on one side an action performed, and on the other side a payment, like [GOT $20], [WATCHED TV], [GOT $0], and [WATCHED BABY], and ask people to spot which cards need to be flipped for violations of the implied social exchange, instead of 25% of people identifying the correct cards according to logic, the number leaps upwards to 65-80%, but, and here’s where things get interesting, that accuracy is related to evaluating violations of fairness rather than violations of logic. Logically, we should check GOT $0 to make sure the other side does not say WATCHED BABY, and WATCHED BABY, to make sure it does not say GOT $0. But if you have social circuitry which has been trained in the hard evolutionary school of constant social interaction, what you’re interested in isn’t logic, it’s catching cheaters.

One of the great problems of social cognition is how to build a neural circuitry that creates a functioning society which prevents people from just bluffing their way to all of the community’s resources constantly without doing any of the work. If you end up on the Sucker end of the stick all the time, you will not be long for the world, and so we have built up strong bits of mental machinery that lurch towards Cheater Detection, and in this case drive us towards the GOT $20 and WATCHED TV cards, because we want to know right away, if somebody got $20 for not doing a job (even though, logically, within the rule we gave, you’re allowed to get $20 for doing other things) or if they got $20 for Watching TV (even though, again, according to the rule, there’s nothing against that). We HAVE to find the cheaters, and are VERY good at making the card selections that will weed them out, while ignoring the ones that won’t.

This is just one example of how Cosmides has probed our latent social circuitry to determine how evolution built brains that allowed us to build civilizations, the tip of a deep iceberg of necessary algorithms which generate the decisions we use instinctively to rate the trustworthiness and dependability of those we have to regularly work with. In 1992, together with her husband John Tooby and Jerome Barkow, Cosmides edited and contributed to The Adapted Mind: Evolutionary Psychology and the Generation of Culture, a fascinating walk through the evolutionary challenges and neural solutions that had to be faced and created to go from our deep primate past into our rich apex species present, in which mere questions as to what ratio our individual genetic inheritance combines with our environment to produce our individual behavior become dwarfed by the larger issue of how we came to have the rich multiplicity of baseline instinctive functionalities that we do, and that bind us all.


Diamond’s autobiography can be found in the sixth volume of The History of Neuroscience in Autobiography which, like the memoir from McGeer in that series mentioned earlier, is also available online. Her 1988 book Enriching Heredity is pretty easy to find copies of, and is engagingly written, clearly communicating her excitement for her topic and the importance of its results for every stage of life, three decades into her project. A good introduction to Cosmides’s work on the neural machinery of social exchange can be found in the chapter she and Tooby wrote for The Adapted Mind, “Cognitive Adaptations for Social Exchange” (1992), and while you’re waiting for that to arrive, her staff page at UC Santa Barbara will lead you to her engaging introductory essay, “Evolutionary Psychology: A Primer,” which entertainingly distinguishes what evolutionary psychology is, and what it is not, what it has accomplished, and what the future holds.

You can also pre-order my History of Women in Psychology and Neuroscience, in which this portrait appears, from Pen & Sword Books this very moment!


bottom of page