Tag Archives: evolutionary psychology

The Goodness Paradox

194 – 184 thousand years ago.

We’re now taking (pre)history ten thousand years at a time.

Earth Abides is an early (1949) entry in the genre of post-apocalyptic science fiction, with some haunting reflections on what it takes to keep a civilization going, or just a human community. In this case the apocalypse takes the form of a lethal infectious disease that wipes out well over 99% of Earth’s human population, leaving scattered survivors to try and put things back together.

The drama is low-key. If you want to read about the remnants of civilized humanity defending themselves against zombies, or venomous man-eating walking plants, or a horde of cannibal anti-nuke zealots, you’ll have to look elsewhere. The threat to civilization in Earth Abides is more subtle. The generation born after the die-off has no understanding of what they have lost, of what the collapsing factories and powerlines and machines around them really were, or how they worked, or how to get them back up and running. The older generation, without the institutions of a complex society backing them up, can’t supply enough discipline and punishment to pass on the arts of civilization. The young will grow up as illiterate scavenger-foragers, skilled with bow and arrow, well-adapted in their own way to a rewilding Earth. Ish, the protagonist, will end his days as the Last American (a fictional counterpoint to the real-life Ishi, the last Yahi Indian).

The community is nonetheless capable of reacting decisively when their survival is threatened.

One day a newcomer enters the scene. Charlie is talkative, forceful, charismatic. The kids adore him. It looks like he might even take over as leader of the little group. But it becomes clear that there is something off about him, even sociopathic. He can turn on the charm, but when thwarted he is menacing. He always carries a gun, but keeps it hidden. He is sexually abandoned. And, in a world without antibiotics, he is infected with a slew of venereal diseases.

Something has to be done about Charlie, and the elders of the group meet to decide what. Their options are limited. Keeping him locked up is not a practical possibility. They could banish him. But who is to say he won’t find a gun, and come back, looking for revenge? They could execute him. But what actual harm has he done, so far? They decide to settle the matter with a vote.

Em located four pencils. Ish tore a sheet of paper into four small ballots.

This we do, not hastily; this we do, not in passion; this we do, without hatred. …

This is the one who killed his fellow unprovoked; this is the one who stole the child away; this is the one who spat upon the image of our God; this is the one who leagued himself with the Devil to be a witch; this is the one who corrupted our youth; this is the one who told the enemy of our secret places.

We are afraid but we do not talk of fear. … We say, “Justice”; we say, “The Law”; we say “We, the people”; we say, “The State.”

Ish sat with his pencil poised … He could not be sure. Yet, at the same time, he knew that The Tribe faced something real and dangerous and even dreadful, in the long run threatening its very existence. … In that final realization, he knew that he could write only the one word there, out of love and responsibility for his children and grandchildren. …

“Give me your slips,” he said.

They passed them in, and he laid them face up before him on the desk. Four times he looked, and he read: “Death … death … death … death.”

Keith Otterbein is an anthropologist who did a study of capital punishment across cultures. He expected to find that capital punishment is limited to complex societies, used to enforce social hierarchy. Instead he found that capital punishment is a universal, present in societies from the simplest to the most complex. It is an option that even the smallest, most easy-going communities – like The Tribe of Earth Abides – may find themselves resorting to. We can infer that for tens or even hundreds of thousands of years, our ancestors have been carrying out group-sanctioned executions of individuals deemed anti-social and a threat to group harmony and survival. This is long enough to have had evolutionary consequences. Long before human beings domesticated the wolf, the wild sheep and goat, the aurochs, we may have been domesticating ourselves, weeding out the wildest and most dangerous from our midst, replacing the old tyranny of the alpha male with the tyranny of Custom and The Law.

The idea that human beings are in some ways like domesticated animals is an old one. It has recently returned to the spotlight. The extraordinary long-term experiment in artificial selection for tameness in foxes carried out by Nikolai Belyaev, Lyudmila Trut, and their coworkers in Russia has demonstrated that selection for tameness ends up selecting for a whole suite of anatomical characteristics as byproducts. Strikingly, many of the features that differentiate Homo sapiens hundreds of thousands of years ago from Homo sapiens today are also features that distinguish tame from wild foxes, and dogs from wolves.

Richard Wrangham’s recent book, The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution connects the dots, setting out one long argument that an evolutionary history of capital punishment has reduced our disposition for reactive aggression (the hot-blooded, spur-of-the-moment, volatile, antisocial kind), while leaving intact our capacity for calculated, cold-blooded, proactive killing (“not hastily, … not in passion, … without hatred”).

Perhaps this explains a very recent finding in human evolution. Apparently 200,000 years ago, we were about evenly matched with Neanderthals, sometimes replacing them, sometimes being replaced by them. By 40,000 years ago however, Neanderthals lose out decisively to modern humans. It may be that what changed in the interim to give us the edge is that we improved our ability to get along peaceably with insiders (including distant insiders we don’t know personally) without losing our ability to apply lethal aggression to outsiders.

Learn This One Weird Trick … (Part One)

… that humans use, and now you can too!

There are people who think that human beings are nothing special. Sure (the argument goes) people have uniquely large brains. But all sorts of creatures have unique features. Elephants are the only animals with trunks. Tamarins and marmosets are the only primates that give birth to twins. Platypuses are the only venomous mammals. Spotted hyenas are the only mammals whose females sport pseudo-penises (through which they give birth!). And so on. If we could ask members of these species they’d claim that they’re the special ones.

But of course we can’t ask them, and in any case, this isn’t a very convincing argument. Human beings have an absolutely outsize impact on the Earth, and the advent of human beings looks like one of the major evolutionary transitions, comparable in importance to the origin of the eukaryotic cell or multicellular life. But even if we buy this, it still leaves open the question of whether there’s a key adaptation – a One Weird Trick – that accounts for the exceptional course of human evolution. Here are some candidates that being are being batted around these days:

1) The cognitive niche. The basic idea is at least as old as Aristotle, that human brings are defined by their capacity for Reason. A modern version of this is advocated by evolutionary psychologist John Tooby and cognitive scientist Steven Pinker. Pinker in particular has elaborated the argument that humans are uniquely adapted to acquire and share knowledge, by virtue of a suite of cognitive, social, and linguistic adaptations. We’ve already touched on several aspects of this: Human beings seem to have taken the capacity for thinking about physical space and retooled it for thinking about the abstract cognitive space of possession – a social relationship. (Other abstract cognitive spaces include kinship, time, and change-of-state.) And humans seem to harness the machinery for processing the sounds of interacting solid objects in creating major categories of phonemes. For a more complete exposition, here’s an academic article by Pinker, and a talk on youtube.

2) Culture. Rob Boyd and Pete Richerson, who’ve done a lot of mathematical modeling of cultural evolution, are skeptical about the “cognitive niche” argument. Too much culture, they argue, is things that have been learned by trial-and-error, and are passed on from one generation to the next without people understanding why they work. Boyd and Richerson appeal, as anthropologists have for generations, to the importance of culture. We mentioned earlier their argument that the frequency of climate change in the Ice Age was nicely calibrated to favor social learning rather than individual learning or instinct. Joseph Henrich provides a recent defense of the importance of culture. Contra Pinker, he thinks humans often don’t have a good cause-and-effect understanding of the things they do, but depend heavily on imitation and the accumulated wisdom of the elders. And see this post, for the importance of High Fidelity cultural transmission in the evolution of animal and human intelligence.

Coming up: Part Two. Recursion and Shared Intentionality

It’s a small world after all

595-563 thousand years ago

The story of human origins is partly a story of Big Things like The Taming of Fire and the The Dawn of Speech. But it’s also the story of some odd byways and quiddities. A nice introduction to some of these is Chip Walter’s book Thumbs, Toes, and Tears: And Other Traits That Make Us Human. (His more recent Last Ape Standing is good too.) Walters considers funny bits of anatomy like our unique big toes and thumbs, and funny bits of behavior like our habits of laughing, weeping, and kissing. Toes and thumbs fossilize, but behaviors can be hard to date, evolutionarily. Presumably these behaviors appeared sometime before modern humans evolved and spread, so let’s pick today’s date. It’s also hard to figure out the exact evolutionary rationale for some of these behaviors. Humor, for example, is not a simple phenomenon: intellectually appreciating a joke, actually finding it funny and enjoying it, and finally laughing, each involve separate areas of the brain.

Laughter, specifically, is a minor human oddity that sheds an interesting light on some big events in human evolution. Robert Provine, a leading laughter researcher, spells out the argument in “Curious Behavior: Yawning, Laughing, Hiccuping, and Beyond.” Chimpanzees have a kind of laugh, a modified vocalized panting synchronized with inhalation and exhalation. Presumably laughter first resulted when panting-during-play evolved into a play signal. But the short bursts of human laughter go further, having freed themselves from synchrony with the inhalation/exhalation cycle. Laughter, in other words, is just one instance of the more general phenomenon of humans having separate controls for vocalization and for respiration. Interestingly, the most prominent examples of complex vocalization – songbirds and some other birds, whales, bats, and humans – are all found in non-quadrupeds. In quadrupeds, breathing is tightly coupled with locomotion: lungs need to be full to stiffen the thorax when the forelimbs hit the ground. Giving up quadrupedalism seems to have allowed for an “adaptive release” in the evolution of vocal abilities in a number of unrelated lineages. So the study of laughter (and other vocalizations) suggests that two key human adaptations – bipedalism and spoken language – are more closely linked than one might have expected.

Another and overlapping set of human particularities involves facial expressions of the emotions. Darwin got a whole book out of this. He concluded (admittedly based on somewhat anecdotal evidence) that different emotional expressions are largely innate. It’s an interesting illustration of his ability to reason from small facts to large conclusions that he also drew a big conclusion about human evolution from this. In Darwin’s day, there were scientists who believed that different human races had evolved from very different prehuman progenitors: one prehuman species giving rise to Europeans, another to Africans, and so on. But Darwin reasoned that the very close similarity in facial expressions (and he had traveled a lot, and witnessed a lot of expressions in a lot of places) and the very similar emotional makeup of humans around the world was evidence that human populations shared a fairly recent common ancestry. Here as in several other cases, a mixture of close reasoning and sheer luck led Darwin to the correct conclusion about evolution long before there was much solid evidence.

Darwin’s work on emotions was neglected for most of the twentieth century by anthropologists favoring a blank slate view of human behavior, but was eventually largely vindicated by a number of researchers, notably Paul Ekman. There is now good evidence for six basic facially expressed emotions: Fear, Disgust, Joy, Anger, Sadness, and Surprise.

If you’re a movie watcher, this list may seem familiar. These emotions (all except for Surprise) are all depicted as little homunculi living inside the head of an 11 year old girl in the animated feature “Inside Out.” Somebody at Pixar Studios knows their Ekman.


So the sappy song is right: There is just one moon and one golden sun, and a smile means friendship to everyone.

Muscles, calories, and curves

1.46-1.39 million years ago

Chimpanzees and humans make different tradeoffs between strength, energy expenditure, and energy storage (i.e. fat). The differences are ordinarily not so evident on the surface, because chimps are also covered with hair. But this video of two hairless chimps, father and son, is (literally) revealing. It shows how buff chimpanzees are: muscular, and with little body fat obscuring muscle definition.

(And yes, it also shows off the chimps’ enormous testicles, adapted to produce lots of sperm and outcompete other males’ sperm in a promiscuous mating system.)

Chimpanzee muscle is 1.35 times stronger than human muscle (a smaller difference than is sometimes reported), with more of the fast twitch fibers the make for bursts of strength, and fewer of the slow twitch fibers that make for endurance.

As a further comparison, here’s figure is from a neat recent paper comparing energy expenditure (TEE or Total Energy Expended) and fat among humans and our closest relations: chimpanzees (genus Pan), gorillas (Gorilla), and orangutans (Pongo). (The numbers are adjusted for differences in overall body mass.)


1.46-1.39 million years ago


This figure is from a neat recent paper comparing energy expenditure (TEE or Total Energy Expended) and fat among humans and our closest relations: chimpanzees (genus Pan), gorillas (Gorilla), and orangutans (Pongo). (The numbers are adjusted for differences in overall body mass.)

What stands out here is that humans are a high energy species. Also we carry a lot more body fat than the other great apes. This applies particularly to women, who need a lot of extra fat to meet the high energy demands of human infants. But it even applies to men. For both sexes, the high energy life style means you want to carry around an extra reserve of fat in case of emergencies.

We don’t know how long ago our ancestors decided to crank up their energy consumption. Maybe back with the rise of Homo erectus (just a few days ago on Logarithmic History). Or maybe later, when the typical modern human pattern of slow maturation was more firmly in place. At some point in the near future, we’ll actually nail down the specific genetic changes leading humans to accumulate more fat, and be able to put a date on the change. It may be that the distinctively human mating system also arose back then, with human females concealing ovulation (no chimp-style monthly sexual swellings) but advertising nubility (with conspicuous fat deposits appearing at puberty).

A high energy life-style also goes with extensive food sharing and changes in human kinship. (Here’s me, on beating Hamilton’s rule through socially enforced nepotism.)

Your cuisinart, a prehistory

A famous movie cut, from 2001: A Space Odyssey, transitions from a bone club, hurled aloft by an australopithecine 2.5 million years ago, to a spacecraft in the year 2001.


Arthur Clarke and Stanley Kubrick, coming up with the plot for the movie/book, were influenced by the popular author Robert Ardrey. In his book African Genesis, Ardrey casts human evolution as a version of the story of Cain and Abel, except in his version the peaceful vegetarians (robust australopithecines) get clobbered by the club-wielding meat-eaters (gracile australopithecines).

We were born of risen apes, not fallen angels, and the apes were armed killers besides. And so what shall we wonder at? Our murders and massacres and missiles, and our irreconcilable regiments?

Ardrey, along with Konrad Lorenz and Desmond Morris, was much in vogue in the 1960s: Sam Peckinpah was another movie director influenced by him. Unfortunately his speculations on evolution and human behavior are probably not of enduring value: he had the misfortune to take up the topic too early to take on board the sociobiological revolution pioneered by William Hamilton, Robert Trivers, and George Williams, and popularized by E. O. Wilson and Richard Dawkins.

Ardrey may not have been off-base in thinking that weaponry and warfare have been an important motive force in human biological and social evolution (moe on this later). But where early stone tools are concerned, a different segue, from Oldowan chopper to Cuisnart may be more appropriate.


Recent research argues that early hominins could have dramatically increased available food energy by pounding vegetables and chopping up meat into more digestible pieces. Tool use may have been an early step in our ancestors’ move to high energy diets. Meat-eating began to be important in human evolution around 2.6 million years ago. Somewhat later we see evidence that some hominins have lighter jaws and aren’t chewing as much. So to celebrate this early dietary revolution, here’s a recipe:

Steak Tartare

Place in a food processor fitted with the metal chopping blade:

1 ½ pounds lean beef (tenderloin, top round, or sirloin) cut into ½ inch cubes

Pulse until meat is coarsely ground, 7-10 seconds. Do not over-process. Remove meat to a chilled platter or individual plates and gently form into 6 individual mounds.

[Optional: Make a spoon shaped indentation on top of each mound and crack into each

1 egg yolk.]

Divide and arrange in small piles around each serving:

½ cup minced onions
½ cup minced shallots
½ cup minced fresh parsley
¼ cup minced drained capers
8-12 anchovies (optional)

Serve immediately and pass separately:

Fresh lemon juice
Worcestershire sauce
Dijon mustard
Hot pepper sauce
Freshly ground black pepper

From The Joy of Cooking 1997

High fidelity

Arms races have been a big engine of evolutionary progress, both in biological evolution and in the evolution of human societies. Another big driver has been improvements in the fidelity of inheritance. We see this in the evolution of genetic systems, including the evolution of life itself, and of the eukaryotic chromosome. And we’ll see it in human social evolution, including the evolution of language, of writing, of the alphabet, and printing.

Both arms races and improved information transmission may have been factors in the evolution of braininess.

jerison brain race

The figure above is from the classic work of Harry Jerison, one of the pioneers in studying the evolution of brain size. It’s several steps away from the raw data, but what it shows is how mammalian Encephalization Quotients (EQs), a measure of brain size relative to body size, evolved over the Cenozoic. The figure might be read as the record of a brainy arms race between prey and predators, leading to increased variance in the EQ bell curve for both.

Primates of course are particularly brainy mammals. One popular explanation for this is a series of arms races within species, with bright monkeys and apes outwitting dimmer ones. This has been called the Machiavellian Intelligence hypothesis (or, in the case of macaques, macachiavellian intelligence).


This hypothesis may not hold up too well, however. One complication is that, contrary to what a lot of evolutionary psychology might suggest, social intelligence in primates is not separate from other sorts of intelligence. The same primate species that are good at solving social problem (e.g. tricking other group members) are also clever about things like tool use and other complex foraging skills. Variation in intelligence across primate species mostly boils down to a single general factor, rather than a bunch of domain-specific aptitudes.

Also, the latest research suggests that variation in diet and ecology, like the distinction between fruit eaters (brainy) and leaf eaters (not-so-much), accounts a lot of variation in brain size, while differences in social complexity (measured by group size) don’t seem to matter.

An alternative to the Machiavellian Intelligence hypothesis is the cultural intelligence hypothesis, with brainier animals more likely to innovate and more likely to learn others’ innovations. The first part pf this equation holds up: across various groups of organisms, including birds and primates, brainy animals are more flexible in their behavior, more likely to discover new adaptive behaviors, and more successful in colonizing novel environments. The second part is trickier. In recent years we’ve learned that learning useful information by observing others (go ahead, call it culture, if you want to annoy anthropologists) is extremely widespread, and found in organisms like guppies and honeybees that no one thinks are terribly bright. So learning from others doesn’t take special smarts.

Where bigger brained animals may excel is not in how much social learning they do, but in how accurately they do it – in copying fidelity. Theoretical models of the evolution of copying suggest that accurate copying makes a big difference. Small changes in copying fidelity can lead to large changes in the persistence of cultural traits. Of course this will crucially important for human evolution: more on this in days to come.

copying fidelity

The expected lifetime, measured in generations, of a cultural trait as a function of the efficiency of social learning (p). Each learning trial uses a new cultural parent drawn from the parent population (see text). Parameter value: n 1⁄4 2.

For a wide-ranging introduction to this rapidly advancing area of research, written by a leader in the field, try Darwin’s Unfinished Symphony: How Culture Made the Human Mind.

Dead baby monkeys

There’s a dark side to being a primate. A few years back a review article summarized data on rates of lethal aggression in non-human animals. The figure below shows some of the results. Several clusters of especially violent species stand out in the figure, including primates (redder is more violent). Bats are pretty nice, though.

dead monkeys

Much of the lethal aggression in primates involves infanticide. Sarah Hrdy demonstrated back in the 1970s that infanticide occurs regularly in Hanuman langurs, monkeys in India. A male who takes over a group of females will systematically kill offspring sired by the previous male. If you think evolution is about the survival of the species, this is hard to explain. But it makes sense given the logic of the selfish gene. Females who lose an infant return more quickly to breeding again, and the father of the next infant is likely to be the killer of the previous one.

Primates may be particularly vulnerable to this grim logic, because they spend a long time as infants. Among primates, commonly,


That is to say that the time, L, a female spends lactating for an infant (during which she is unlikely to conceive), is usually greater than the time, G, she spends gestating an infant. This puts particular pressure on males to hurry things along by eliminating nursing infants fathered by other males.


As a result, infanticide is relatively common among primates, and females under particularly strong pressure to find ways to avoid it. Hanuman langurs live in one-male units, where a female has little choice about who she mates with. In other species, by contrast (most baboons, chimpanzees), multiple males reside with multiple females. In these species females are often sexually promiscuous, sometimes actively soliciting multiple males for sex. This is probably mostly a matter of confusing paternity sufficiently to suppress the threat of infanticide. There’s a general lesson here: females are not always monogamously inclined, but female promiscuity generally has different evolutionary roots than male promiscuity.