Do Harsh Environments Trigger Early Puberty? Using Historical Data to test Evolutionary Hypotheses

–  by Tony Volk

In 1993, Jay Belsky and his colleagues noticed that girls who lacked fathers and/or grew up in harsher environments seemed to also have early puberty. In one of the first evolutionary developmental hypotheses since Bowlby, they proposed that girls might be maturing faster in harsh environments as a way of reproducing before they died in those harsh environments. These sorts of trade-offs are known as life history theory (LHT).

Psychologists have been studying LHT for 30 years and recent summaries of the pubertal LHT data showed that there was a small, but statistically significant, relationship between growing up in a harsh environment and early puberty (for both boys and girls). While it relied on correlational data (that can’t determine between cause and effect), it nevertheless seemed like a promising example of a successful evolutionary developmental hypothesis. However, there was a problem. Data from evolutionary anthropologists, who mostly looked at poorer countries, didn’t support this LHT pubertal relationship. A survey of evolutionary researchers showed that this was one of the biggest points of disagreement between evolutionary psychologists and anthropologists. I decided to try and break the tie by looking at historical (and hunter-gatherer) data.

Why does historical behavioral matter if we’re interested in explaining behavior today? The reality of evolution is that it works in a forward direction. It solves today’s problems tomorrow by filtering which genes get passed into future generations. Thus, in order to understand the adaptations we have today, we have to look into the past to understand what problems they solved for our ancestors. For example, we crave sugar and salt today not because that’s adaptive in the modern world (it’s not!), but because it was adaptive in the past when those valuable resources were scarce. This is important because modern environments pose many new challenges while eliminating many older challenges. Similarly, modern hunter-gatherers (who are not necessarily the same as past hunter-gatherers) can shed some light on the kinds of challenges and opportunities that humans may have faced in the past when most people lived as hunter-gatherers. To piece together the best picture of what harsh environments had to do with LHT pubertal acceleration in our evolutionary past, I gathered a variety of cross-cultural data: historical texts and records, skeletal remains, forensic and medical science, archaeological artifacts, DNA lineages, and hunter-gatherer data.

To start with, the past experience of “harsh” was very different. In earlier research I had shown that almost half of all past humans died before puberty- that’s seriously harsh! Growing up, those who survived would have regularly witnessed others dying of disease, hunger, or violence. In contrast, children in developed countries have a 1-2% chance of dying before puberty- that’s a lot lower! What I view as perhaps humanity’s greatest achievement was a challenge for psychologists who wanted to measure harsh environments in modern, developed countries where disease, hunger, and violent deaths are far less common. Indeed, most psychologists use indirect cues that ranged from father absence (as a proxy for low paternal care), to moving frequently (a source of uncertainty), to not eating out in nice places or having old sneakers (indicating low resources). The reality is that these modern cues just don’t map onto the highly visible and valid cues in the past. What’s more, in the past harsher environments invariably meant fewer resources. The historical poor had no free schooling, medicine, housing, or food. Past children born in harsher environments were more likely to die of starvation, disease, or violence- three closely intertwined causes of mortality.

So how could a child in harsher environments speed up their puberty? Puberty costs hundreds of thousands of calories. Preparing the female body for pregnancy and delivery costs hundreds of thousands more, with expensive lactation to follow. In the face of war, poverty, and/or disease, where did these extra calories come from? To paraphrase an old commercial, “Where’s the beef”? The reality was that there wasn’t any extra food for the historically poor and/or powerless. They could trade off adult size for earlier development, but that would only put them at risk for worse pregnancy and infant outcomes as well as put them at a severe competitive disadvantage with any adults who did not sacrifice ultimate growth for speed. In contrast, in the past, the rich and powerful could afford extra food, health advantages, and better security. In essence, they could afford to pubertally accelerated life histories. So did they?

Yes, they did. Faced with less harsh (and less unpredictable) environments, historical and hunter-gatherer elites accelerated their growth and menarche, reproduce earlier due to greater calories and looser social rules, enjoyed increased fertility due to energetic and behavioral choices, and they co-opted other adults to care for their larger broods (by paying or enslaving them). In women, perhaps the clearest example is the wealthy’s use of wet nurses. This saved historically wealthy mothers calories and allowed for shorter interbirth intervals while imposing energetic and reproductive costs on the poorer wet nurses. In men, history shows that historical men often translated wealth into polygyny, sometimes achieving incredulous levels of reproductive success that left a genetic footprint on entire populations (e.g., 1% of the world or 8% of Asians, are descendants of Genghis Khan and his sons).

To summarize, historical (and hunter-gatherer) data consistently showed the opposite pattern as we see in modern populations. Across history, cultures, and geography, the wealthy and powerful translated their status and resources into earlier puberty and increased fertility while the poor lived in harsher environments and have delayed puberty and reduced fertility. This casts doubt on the causal feasibility of the modern patterns as being an adaptation to the past. I believe that these data not only shed light on this one aspect of Life History Theory, but that they also show how important understanding historical data is for evolutionary research. We are adapted to solve yesterday’s problems, so for anyone interested in evolutionary origins of human behavior today, my advice is to start by examining our evolutionary past.

Read the original article: Volk, A.A. (2023). Historical and hunter-gatherer perspectives on fast-slow life history strategies. Evolution & Human Behavior, 44(2), 99-109.