John Tooby on Dysgenics

Is mutational load behind the birth dearth?

The late John Tooby in 2016 wrote,

Of the roughly 5 ½ billion people who survived past puberty, perhaps only one billion would be here were it not for modern sanitation, medicine, technology, and market-driven abundance. Ancestrally, the overwhelming majority of humans died before they had a full complement of children, often not making it past childhood.

This is good news for the living individuals, but bad news for the evolution of the species.

a number of methods have converged on the estimate that every human child contains roughly 100 new mutations—genetic changes that were not present in their parents. To be sure, many of these occur in inert regions, or are otherwise “silent” and so do no harm. But a few are very harmful individually; and although the remainder are individually small in effect, collectively they plague each individual with debilitating infirmities. 

…If a species is not to melt down under the hard rain of accumulating mutations, the rate at which harmful mutations are introduced must equal the rate at which selection removes them (mutation-selection balance).

The human condition used to weed out harmful mutations. But modernity has shifted the balance.

Most ancestral humans were fated by physics to be childless vessels whose deaths served to carry harmful mutations out of the species.

Now, along comes the demographic transition—the recent shift to lower death rates and then lower birth rates. Malthusian catastrophe was averted, but the price of relaxing selection has been moving the mutation-selection balance toward an unsustainable increase in genetic diseases…Indeed, it is possible that the drop in death rates over the demographic transition caused—by increasing the genetic load—the subsequent drop in birth rates below replacement: If humans are equipped with physiological assessment systems to detect when they are in good enough condition to conceive and raise a child, and if each successive generation bears a greater number of micro-impairments that aggregate into, say, stressed exhaustion, then the paradoxical outcome of improving public health for several generations would be ever lower birth rates. One or two children are far too few to shed incoming mutations.

Consider my mother’s case. She was born 101 years ago today. She died at age 53 of lung cancer, probably caused by smoking. Had she been born in 1943 rather than in 1923, she would have had more children and lived longer (because smoking would have gone out of fashion by the time she was 20).

Her blood type was Rh positive, and mine was Rh negative. Doctors discovered the Rh factor less than 15 years before I was born, in 1954. My survival was a miracle of modern medicine. The RhoGam shot, which would have enabled her to have more children, was not developed until 10 years later.

Anyway, whatever mutations I carry, for good or ill, would not have been carried by a similar human born prior to 1950. That individual would have died in infancy.

Today, we are surrounded by people with genetic profiles that are relatively novel. A few of these are in some sense “better” than the historical human average, but if Tooby is correct, then most are bound to be worse. Similarly fragile profiles would not have been found in people 200 years ago, because the oddities would have been weeded out earlier by natural selection Modernity has intervened against the evolutionary process.

Of course, even prior to the Industrial Revolution, human culture found ways to intervene with evolution. There is the theory that we have “self-domesticated” by culling the most violent among us.

But reading Tooby’s piece has me pondering the notion that the genetic mix among humans could change swiftly and dramatically in modern times. In addition to adverse physical traits that could survive, adverse psychological traits also could face less selection pressure.

Suppose that for most of human history, people with mildly fragile psychological tendencies were outbred by people who were more robust. But starting a few generations ago, the mildly fragile were able to survive. As they remained in the gene pool, the prevalence and severity of psychological fragility would tend to increase. (By psychological fragility I do not mean some specific syndrome or disorder, but any of many possible issues.) The potential increase could be quite dramatic.

If trends in recent decades have been dysgenic, then this could turn out to be self-correcting. The birth dearth may be part of such a process. Perhaps the people who are having families have fewer harmful mutations, so that dysgenics will reverse in future generations. But perhaps not.