The less intelligent are more fertile — so why aren’t humans becoming dumber over time?

A new test of 5.9 million texts spanning years 1850 to 2005 suggests that genetic factors of intelligence have been declining as environmental influences have been improving.

In a new study led by Michael A. Woodley of Menie, of the Technische Universitat Chemnitz, Germany, and the Vrije Universiteit, Belgium, researchers assessed the changing prevalence of words known to be associated with intelligence. The research was published in April in the journal Frontiers in Psychology.

While most measures of IQ have been criticized over the years, many studies have consistently suggested greater fertility among those who are “less intelligent.” This led early theorists to propose that humans were becoming less intelligent over time.

A common argument goes that until the mid-nineteenth century, individuals with higher wealth, education, and socioeconomic status (all thought to indicate more intelligence) were more likely to have more surviving offspring. In other words, intelligence used to confer an evolutionary fitness benefit. In modern times, however, some have suggested that increased global temperatures, reduced environmental harshness, improved agriculture, more advanced nutrition and medicine, including birth control and reduced child mortality, and greater levels of social welfare, schooling, and universal healthcare may have all contributed to, as the authors put it, “a demographic transition characterized by general reductions in fertility, which were more pronounced among those with higher intelligence.”

In contrast to these theories, however, recent data has suggested the opposite: On average intelligence has been increasing over time. These conflicting notions have been reconciled by the hypothesis that genetic decreases in intelligence may be more than offset by improvements to environmental factors, such as greater access to education, improved nutrition, and hygiene.

The authors tested this combined hypothesis by comparing the frequency of words taken from the WORDSUM vocabulary test, a strong and reliable measure of intelligence. More difficult words, which are associated with heritable components of intelligence, have declined in usage since 1850, but easier words, which are more associated with environmental effects, have increased in usage, even after controlling for increasing literacy rates and the age of each word.

“These findings provide compelling evidence for the co-occurrence model,” the authors report, suggesting that heritable general intelligence may be falling while environmental factors are improving intelligence overall.