The genes that predispose people to attain higher levels of education have been in decline over the past 80 years, and researchers are suggesting that they're now under negative selection, which could have a big impact on our species in the coming centuries.

A study involving more than 100,000 people in Iceland found that those who carry the genes for longer education time were less likely to have a big family, which means the smartest people in the room were actually contributing less to the Icelandic gene pool.

"As a species, we are defined by the power of our brains. Education is the training and refining of our mental capacities," said Kari Stefansson, CEO of Icelandic genetics firm deCODE, which ran the study.

"Thus, it is fascinating to find that genetic factors linked to more time spent in education are becoming rarer in the gene pool."

To be clear, this does not necessarily mean that humans are getting dumber - we're going to need a whole lot more evidence to get anywhere near a conclusion like that.

There's also the fact that more people are getting access to education than ever before, so even if less educated people are having more offspring, non-genetic factors like more schools could counteract and even eclipse the effect.

But if we look at the trend over the course of several centuries into the future - well beyond the proliferation of schools and training access - the researchers say it could have a significant effect on our species in the long run.

"It is remarkable to report changes … that are measurable across the several decades covered by this study," the study concludes. 

"In evolutionary time, this is a blink of an eye. However, if this trend persists over many centuries, the impact could be profound."

The researchers analysed the birth rate of 129,808 individuals born in Iceland between 1910 and 1990 who had their genomes sequenced, and compared this to their education levels.

They found that there was a genetic factor related to a person's likelihood of attending school for longer, and came up with a 'polygenic score' based on 620,000 sequence variations - or markers - in the human genome to determine an individual's genetic propensity for education.

As the team points out, no one knows the exact mix between genetic and environmental factors that leads to someone's education level, but previous studies have estimated that the genetic component of educational attainment can account for as much as 40 percent of the difference between individuals.

Once that polygenetic score was correlated with factors like educational attainment, fertility, and birth years, the researchers found that those with a higher genetic propensity towards more education tended to have fewer children.

They also found that the average polygenetic score has been declining at a small, but significant rate on an evolutionary timescale.

As Ian Sample reports for The Guardian, the team found a drop in IQ of about 0.04 points per decade, but if all the genetic factors that could be linked to education were taken into account, that figure would increase to 0.3 points per decade.

Interestingly, the link between a higher propensity towards more education and having fewer children wasn't because going to university is hard, and eats into your family-raising time - the team suggests that the genes involved in education can also affect human fertility on a biological level.

Because even those who carried the genes for longer education time, but who did not actually get more education, still had fewer offspring on average than those without the genetic factor.

"Those who carried more 'education genes' tended to have fewer children than others," Sample explains.

"This led the scientists to propose that the genes had become rarer in the population because, for all their qualifications, better educated people had contributed less than others to the Icelandic gene pool."

Again, this is all speculation is only based on one country, and it's incredibly difficult to predict what's going to happen to humans in the distant future.

But it's certainly something to keep an eye on, the researchers say, and if anything, highlights the importance of a continued effort towards ensuring that every human has access to education, because that can override the negative selection that appears to be in play.

"In spite of the negative selection against these sequence variations, education levels have been increasing for decades. Indeed, we control the environment in which these genetic factors play out: the education system," Stefansson said in a press statement.

"If we continue to improve the availability and quality of educational opportunities, we will presumably continue to improve the educational level of society as a whole. Time will tell whether the decline of the genetic propensity for education will have a notable impact on human society."

 The study has been published in Proceedings of the National Academy of Sciences.