CONNECTIONS (#2 of 6): Parsing the Human-Neanderthal Puzzle, With Help From Hart and Risley



It’s de riguer in evolutionary biology theorizing that behavior is integral to the survival of a species. But unless you understand behavior you can’t fully appreciate that nuances embedded in that notion. Indeed, when other disciplines invoke behavior they often greatly understate its power and complexity. Bear with me as I unpack an example.

Demise of the Neanderthals

During the past 7 million years or so, our planet always supported multiple species of hominins — until recently, that is. Once homo sapiens burst onto the scene, the days of other hominins were numbered. That’s the descriptive fact, and scientists have been scrambling to figure out why. Much of the most breathless reporting on this mystery concerns our close relatives, the Neanderthals, who coexisted with us for a time in Europe and Asia. Writes Longrich (2024):

Why did humans take over the world while our closest relatives, the Neanderthals,  became extinct? It’s possible we were just smarter, but there’s surprisingly little evidence that’s true…. We weren’t that different. But we met Neanderthals many times, over many millennia, always with the same result. They disappeared. We remained.

A quick recap of Longrich’s premise: Neanderthals used to be portrayed as primitive brutes — the filthy, grunting cavemen of film stereotypes, an evolutionary dead end whose backwardness guaranteed their demise.

But we now know that Neanderthals were, like early homo sapiens, tool makers, artists, skilled hunters — and most likely practitioners of verbal behavior. As our view of Neanderthals has come to increasingly approximate our own mirror image, it has become more and more difficult to explain why the arrival of homo sapiens in Europe presaged their extinction.

A lot of speculation has focused on biological characteristics. Maybe cold-adapted Neanderthals were wiped out by climate change. Maybe humans benefitted from a more varied and flexible diet. And so forth.

When behavior is invoked to explain what happened to our hominin cousins, the explanation is either hopelessly vague (e.g., humans were “smarter” and “outcompeted” their Neanderthal cousins) or oddly specific (e.g., humans were better at killing).

“More Brains”

Nicholas R. Longworth, of University of Bath, advances a hypothesis that cuts a little closer to how behavior really works. It’s thought, based on the example of contemporary hunter-gatherer societies, that both Neanderthals and early humans traveled in relatively small bands that were knit by kinship and shared language and culture. But Longworth posits that, for varied reasons that need not be recounted here, human bands tended to be larger. From this, Longworth proposes that what gave humans the advantage over Neanderthals in an unforgiving, resource-poor environment, was in effect a richer culture.

Larger bands have more brains. More brains to solve problems, remember lore about animals and plants, and techniques for crafting tools and sewing clothing. Just as big groups have higher genetic diversity, they’ll have higher diversity of ideas.

Longworth’s point is that in social-verbal groups there is the potential for “crowd-sourced intelligence,” (this is, by definition, true and indeed the entire basis for the concept of culture). When individuals are socially networked in this way, each person can benefit from the behavioral history of each other person.

Network Connections: Metcalfe’s Law

The basis for Longworth’s postulate is Metcalfe’s Law, which states that the number of connections in a network depends on the number of interconnected individuals. For example, as the graphic below illustrates, as a network shifts from 1 to 5 to 10 members, the number of connections rises from 1 to 10 to 36 (with n = number of participants, the number of connections = n/(n-1)/2].

A summary of Metcalfe’s law in the context of social networks. Reproduced from The Reliants Project under the terms of a Creative Commons Attribution 4.0 International License. Original at

In a tribe where everyone knows everyone, and each person is a node in the “social network,” Metcalfe’s law is exactly right: As group size grows, each person will interact with, and potentially learn from, more other people. As Longworth writes:

Information flows through these connections: news about people and movements of animals; toolmaking techniques; and words, songs and myths. Plus the group’s behaviour becomes increasingly complex. Consider ants. Individually, ants aren’t smart. But interactions between millions of ants lets colonies make elaborate nests, forage for food and kill animals many times an ant’s size. Likewise, human groups do things no one person can – design buildings and cars, write elaborate computer programmes, fight wars, run companies and countries.

The Hart and Risley Connection

I don’t question Longworth’s basic proposition about the value of larger groups, but it overlooks something crucial about how individual behavioral repertoires are formed. For a simple insight on this front, we need only consider the famous Hart & Risley study described in the 1995 book Meaningful Differences in the Everyday Experience of American Children, which recently has been re-popularized in Dana Suskind’s (2015) Thirty Million Words: Building a Child’s Brain. The take-home message of that study; Beginning essentially from birth, the amount of language experience a child accumulates predicts early language development, school readiness, later language skill and IQ, and more. Hart and Risley observed three cohorts of children whose parents talked to them at dramatically different rates. By the age of 3, it was estimated that the language “haves” had heard 30 million more words than the language “have nots” (left panel, below). Unsurprisingly, amount of language experience correlated with growth in child language ability (right panel).

Hart and Risley observed three cohorts of children whose parents talked to them at dramatically different rates. By the age of 3, it was estimated that the language “haves” had heard 30 million more words than the language “have nots” (left panel, below). Unsurprisingly, amount of language experience correlated with growth in child language ability (right panel).

Key findings of Hart and Risley (1995). Left: Words spoken to children in three groups defined by socioeconomic status but differing in amount of caretaker verbalizing. Reproduced from Right: Differential growth in vocabulary size in the three groups. Reproduced from

Now let’s return to the different-sized tribes of early humans and Neanderthals. Let’s also, for the sake of simplicity, assume that the two species possessed exactly the same biologically-gifted capacity for verbal behavior, such that they would benefit identically from the same language experience. It stands to reason that in tribes consisting of more people, a child (a) will be exposed to more talking, and (b) can engage verbally with more different people. This differential rate of language “input” could easily fuel different amounts of language development in a child.

That’s not all. An unacknowledged likelihood in the findings of Hart and Risley is that children who benefit from a rich language environment don’t just learn more words. They develop richer networks of stimulus relations. Speaking loosely, the more talk there is, the more different things may get talked about, creating the opportunity to relate more things to one another. This isn’t something that Hart and Risley discussed, but should be obvious to anyone who knows even a bit about stimulus relations. Metcalfe’s Law applies as much to individual-organism stimulus classes as it does to social networks. For nodes, substitute “bits of knowledge” for different people, and for interpersonal connections, substitute stimulus relations. Voila! for each individual, richer, better elaborated stimulus classes reflecting deeper understanding on all topics.

Of course, these being stimulus classes and the organisms involved being human, a side effect of those enriched classes will be lots of derived stimulus relations — “free” learning which emerges spontaneously as a side effect of directly-practiced stimulus relations. In effect, people will “know” things they haven’ been taught, perhaps including things that no member of the tribe was capable of teaching. And the bigger the stimulus class, the more derived relations. Moreover, because every individual has a different verbal history, different people will derive different relations, meaning that the bigger the group the greater the capacity for the creative or insightful thinking this implies.

And there’s one more thing. Because no two members of a tribe will have exactly the same verbal repertoire, their contributions to others’ verbal development will be different. For verbal interactions fueling the development of a given stimulus class, this potentially means more varied contributions to naturally-occurring multiple-exemplar training — that is, each node and each relation in a child’s given stimulus class will be based on more exemplars. This means that what’s in those stimulus classes, including the derived relations, becomes more generalizable to novel situations

Overall, with more cultural conspecifics and a more verbal milieu, members of the tribe become more flexible thinkers who are better able to adapt to changing circumstances. And the more new problems they solve, the more there is for the tribe to talk about, to transmit culturally across generations. As in the Hart and Risley study, the verbally rich get richer.


Given that the critical evidence lies buried in the distance past, there’s no way to directly test Longworth’s postulate about relative group size in humans and Neanderthals, much less the implications for behavioral differences that could have affected the two species’ long term survival. And we must acknowledge that human beings have a way of manufacturing just-so stories in which their planetary dominance was deserved and foreordained. I might be engaging in a bit of that fallacy — it just might be, in fact, that the survival of humans and demise of the Neanderthals were evolutionary accidents. Run the tape of history a second time and the outcome might be different. We’ll never know.

But we do know a lot about how individuals learn, and Longworth’s postulate provides ample raw material for speculation. Indeed, it’s reasonable to think that humans and Neanderthal children could have been like the high and low socioeconomic-status children, respectively, in Hart and Risley’s study. Let differential developmental trajectories play out across multiple generations, pair this with cultural transmission of skill and knowledge, and you could end up with vastly different intellectual toolkits in the two species, even given identical biological gifts. For contemporary circumstantial evidence, look only to the exciting literature showing that practicing relational repertoires raises children’s IQ scores.

Remember, though, that we don’t (and probably can’t) know whether human and Neanderthal brains were relationally equivalent. We know that among contemporary species humans are precociously relational in ways even their closest living relatives, the great apes, can’t approximate. We also know that hominin brain size expanded dramatically about 800,000 to 200,000 years ago, a development that benefited both humans and Neanderthals. It’s tempting to think that brain size correlates with relational potential, but of course it’s possible that early humans humans were relational rocket aces compared to Neanderthals, in which case the Longworth postulate becomes less compelling.

The Connection That Most Matters

Let’s not, however, let all of this equivocating obscure the bigger picture. Many other disciplines invoke behavior to help solve vexing problems, but their accounts rarely are as nuanced and sophisticated as the ones we could lend them. And, unfortunately, members of those disciplines typically aren’t going to go looking for us for assistance. Indeed, due to the “world of our own” that Skinner popularized, the one in which we inhabit discipline specific journals and organizations, people in other disciplines might not even know we exist. Or if they do, it might not be obvious to them that we have something to offer. As Pat Friman has written:

Skinner’s vision of behavior analysis was that it would become a mainstream science relevant to all aspects of human behavior and that it would be harnessed to improve the human condition substantively. Alas, that vision has yet to materialize…

Among the problems: Large swaths of our research literature focus on the behavior of nonhumans and humans with rare conditions, often in idiosyncratic contexts, such that the generalizability of the findings could, to an outsider, seem limited. For instance, Friedman and Fisher (1998) opined that:

Fundamental laws of reinforcement were derived [from] the behavior of captive starved lower animals. Such laws are … generalizable to the conditioning of single captive starved lower animals and mentally retarded and very young human students, but not beyond.

Friman specified the antidote:

To achieve mainstream status, behavior analysis needs to compromise neither its principles nor its practices. A much more practical and efficient way to enter the mainstream is to integrate with a field that is already there.

In other words, if you want behavior analysis to have an impact in Discipline X, you’d better start working in Discipline X. Friman himself has addressed a number of problems in pediatric medicine and, for the most part, found a friendly audience. His take-home message: People in other disciplines want tools to help solve the problems that concern them. However, as the old saying goes, you may have to bring the mountain to Mohammed.

It’s striking just how comfortably behavior principles map onto, and amplify, Longworth’s conjecture. But until we have behavioral evolutionary biologists and behavioral paleontologists, the world may never know. Yeah, of course there are hurdles to creating hybrid scientists like that — where, for instance, could one obtain the right combination of training experiences? And trailblazers in those hybrid domains could, at first, have a difficult time gaining credibility for their contributions. Yet although science is always hard, when behavior analysts have made the effort they have always found ways to tackle new kinds of problems. If the proposed hybrid specialities see like too much of a stretch to you, therefore, it’s YOU who is underestimating the power and scope of behavioral analyses.