IQ scores are on a decline, reversing a trend that saw scores rising at a steady rate during the 20th century, according to a new study.

Researchers from the Ragnar Frisch Centre for Economic Research in Norway studied the IQ scores of about 730,000 Norwegian men born between 1962 and 1991. They found that scores grew by nearly three percentage points every decade for people born between 1962 and 1975. But among those born after 1975, scores fell.

The cause of the IQ decline is due to environmental factors, and not genetics, said Ole Rogeburg, a senior research fellow at Ragnar Frisch Centre and co-author of the study on IQ scores, published in the Proceedings of the National Academy of Sciences journal.

“It’s not that dumb people are having more kids than smart people, to put it crudely,” Rogeburg told CNN. “It’s something to do with the environment, because we’re seeing the same differences within families.”

Environmental factors include differences in the way young people are educated, increases in time spent online, changes in nutrition and less reading overall.

The downward trend is a reversal of the Flynn effect, a term that describes the vast improvement of IQ scores in many parts of the world throughout the 20th century. According to the study, the Flynn effect peaked in the mid-1970s and has declined in the decades since.

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Write to Mahita Gajanan at mahita.gajanan@time.com.