Are humans degenerating genetically and getting dumber as a result?

Each person is born with approximately 100 genetic mutations not present in their parents, and they will pass on about half of these to their offspring. This cycle continues with each generation, prompting the question: are harmful mutations accumulating over time, leading to a decline in human physical and mental capabilities?

Some experts believe this is the case. Geneticist Michael Lynch, in a 2010 publication, predicted a significant decline in human fitness in the coming centuries within industrialized societies. Around that time, several studies indicated a drop in IQ scores in countries like the UK, Australia, Denmark, Sweden, and Norway, suggesting a potential decrease in human intelligence.

The concept of human degeneration was historically associated with unethical eugenic policies in the 20th century. While these early theories were often baseless and prejudiced, modern advancements allow scientists to sequence genomes and directly measure mutations to understand the true scenario.

Research reveals that humans experience a higher mutation rate compared to most animals. The primary factor is paternal: unlike women who are born with a set number of eggs, men produce sperm continuously from stem cells that mutate over time. As men can father children over several decades, this increases the potential for mutation accumulation compared to shorter-lived species.

Most of these new mutations are harmless, as much of human DNA is non-functional. However, some mutations can be detrimental, potentially affecting protein-coding genes or regulatory sequences that control gene activity.

While severe mutations can be fatal, those with minor negative effects can persist across generations. A key question is how populations avoid the continuous accumulation of harmful mutations.

Traditionally, genetic theory suggests that some individuals, by chance, inherit more harmful mutations than others. These individuals are less likely to reproduce, which keeps the “genetic load” of harmful mutations stable.

This balance, however, can shift. In the past, about half of all children died before reaching adulthood, but today, due to vaccinations and abundant food, most children in wealthier countries survive. This “relaxed” natural selection may allow harmful mutations to accumulate, as Lynch proposed, potentially reducing fitness by 1 to 5 percent per generation.

This scenario poses a potential issue, but Lynch’s conclusions largely stemmed from studies on animals like flies and worms. To explore this in mammals, Peter Keightley at the University of Edinburgh conducted research on mice. His team bred 55 lines of mice over 21 generations in optimal conditions, simulating relaxed selection.

The findings, published in 2024, indicate a fitness reduction of less than 0.4 percent per generation in humans, with Keightley suggesting the actual impact could be even smaller.

Natural selection continues to act on humans, as evidenced by the fact that at least a third of conceptions end in miscarriage. “There’s always selection,” notes Joanna Masel from the University of Arizona.

Being less fit isn’t always a bad thing

In evolutionary terms, greater fitness isn’t invariably beneficial. Infections have historically driven high child mortality, and while some gene variants offer resistance, they can also lead to conditions like sickle cell disease. As Masel points out, “If there’s no malaria, you really don’t want them.”

Similarly, while past gene variants helped combat starvation, they may be maladaptive in times of food abundance.

Masel suggests that while evolution can eradicate nearly all harmful mutations in organisms with small genomes and large populations, such as bacteria, this isn’t feasible with humans.

“Our genomes are monstrously bloated with all kinds of parasitic elements,” she explains. “There’s more deleterious mutations coming in than we can get rid of. But we have ways of compensating for them.”

Rather than addressing each genetic issue individually, organisms develop systems akin to sewage systems to handle multiple issues concurrently, Masel says. Notably, rare beneficial mutations with significant effects can counterbalance numerous slightly harmful ones, as major harmful mutations are quickly eliminated.

This concept has significant implications. “Deleterious mutations may be the driving force of complexity, because they create the mess that needs to be cleaned up at higher levels of complexity,” explains Masel. For example, when mutations introduced junk DNA into genes, cells developed mechanisms to remove these segments from RNA copies of genes.

Fascinatingly, simulations by Masel’s team suggest that when mutation rates rise, beneficial mutations accumulate more rapidly than harmful ones.

“You’re actually improving the garbage disposal system faster than you’re creating more mess,” Masel states. “The math counterintuitively, to our surprise, came out that way.”

If this holds true, the elevated mutation rate in humans may not pose as significant a threat as previously thought, and studies indicating declining IQ could be coincidental. While the science is not fully settled, it appears there is no urgent need to worry about human degeneration. This is fortunate since reversing it would be extremely challenging.

Meanwhile, Masel suggests there are more pressing concerns, such as climate change, where the science is conclusive and demands urgent attention. I fully concur.