Are humans degenerating genetically and getting dumber as a result?

You are a mutant. You were born with a hundred mutations that your parents didn’t have. You will pass on about half of them to your children, if you have any, who will have 100 new mutations themselves. And their children too, etc. So, are we humans accumulating harmful mutations generation after generation, leading to a decline in our physical and mental fitness?

Some think so. “A substantial reduction in human fitness can be expected over the coming centuries in industrialized societies,” wrote geneticist Michael Lynch in 2010. At that time, several studies reported a decline in IQ in a number of countries, including the United Kingdom, Australia, Denmark, Sweden and Norway. It seemed like this might be direct evidence that we were getting even dumber.

The idea of ​​human degeneration was, of course, the basis of extremely unethical eugenics policies in the 20th century. There is a very bad story here. But while early eugenicists largely made things up to justify their prejudices, it is now possible to sequence genomes and measure mutations directly to see what is really happening.

This shows that humans have a relatively high mutation rate compared to most other animals. Fathers are the main problem: while women are born with their eggs already formed, in men, sperm are continuously generated from stem cells that mutate over time. Since humans can father children over several decades, mutations have more time to accumulate than in shorter-lived species.

Today, most of the 100 or so new mutations we have make no difference because most of our DNA is junk. But some may be harmful. They can occur in a protein-coding gene, resulting in a defective protein, or in a regulatory sequence, changing the activity of the gene.

Severe mutations kill individuals unfortunate enough to contract them. But mutations that have only a minor harmful effect can be passed down from generation to generation. So what stops more harmful mutations from accumulating in the population?

The conventional idea in genetics is that, by chance, some offspring end up with much more harmful mutations than others. These individuals will be more likely to die before they can reproduce, or may be unable to reproduce. This horribly unfair process stabilizes the “genetic load” of harmful mutations at a certain level.

However, this level may change. About half of children used to die before adulthood, but in high-income countries almost all now survive thanks to vaccines, abundant nutrition, and more. This relaxed natural selection causes harmful mutations to accumulate, Lynch suggested, leading to a reduction in people’s fitness of at least 1 percent per generation and perhaps as much as 5 percent.

This would be a serious problem. But some of the studies on which Lynch’s conclusions were based were done on animals like flies and worms. So, Peter Keightley from the University of Edinburgh in the United Kingdom decided to measure the accumulation of mutations in a mammal. His team bred 55 lines of mice over 21 generations under favorable conditions, that is to say with relaxed selection.

The results, published in 2024, would amount to a reduction in fitness of less than 0.4% per generation in humans, and Keightley thinks there are many reasons why this reduction would be much less in reality.

To begin with, natural selection continues to act on humans. For example, at least a third of conceptions result in miscarriage. “There is always a selection,” says Joanna Masel of the University of Arizona.

Being less fit isn’t always a bad thing

Additionally, fitness in the evolutionary sense is not always desirable. Infectious diseases have been a leading cause of high child mortality in the past and still kill many children in some areas, but genetic variants conferring resistance to these diseases can have big downsides – the classic example being those that protect against malaria but cause sickle cell disease. “If there’s no malaria, you really don’t want it,” says Masel.

Starvation and malnutrition were also major causes of mortality in the past, but the genetic variants that prevent them are likely also often maladaptive when food is plentiful.

More generally, Masel thinks that while evolution can eliminate almost all harmful mutations in organisms such as bacteria – which have tiny genomes and huge populations – this simply isn’t possible in humans.

“Our genomes are monstrously inflated by all kinds of parasitic elements,” she says. “There are more deleterious mutations than we can eliminate. But we have ways to compensate for them.”

Essentially, rather than trying to clean up each genetic “mess” individually, organisms develop the equivalent of sewage systems to continually clean up many messes, Masel explains. In biological terms, what has been overlooked is that rare beneficial mutations with a large effect can offset many slightly detrimental mutations. (Remember that rare mutations with a significant harmful effect are quickly eliminated.)

This idea has profound implications. “Deleterious mutations can be the driver of complexity, because they create a mess that needs to be cleaned up at higher levels of complexity,” says Masel. For example, when mutations filled genes with fragments of junk DNA, cells evolved a system to remove these junk fragments from the genes’ RNA copies.

Intriguingly, simulations conducted by his team suggest that as mutation rates increase, beneficial mutations accumulate faster than harmful mutations.

“You’re actually improving the waste disposal system faster than you’re creating more damage,” Masel says. “Counterintuitively, to our surprise, the calculations worked out like this.”

If this is true, the higher mutation rate in humans may not be the big problem that many biologists assume, and studies reporting lower IQ could simply be a coincidence. The science isn’t settled yet, but it seems there’s no reason to panic about the degeneration of humans – which is just as well as there wouldn’t be an easy way to reverse the trend.

In the meantime, there are other things we should be much more concerned about, Masel says. “I think there are things, like climate change, where the science is settled and we should panic,” she says. I completely agree.