IQ scores have been steadily falling for the past few decades, and environmental factors are to blame, a new study says. Norwegian researchers analyzed the IQ scores of Norwegian men born between 1962 and 1991 and found that scores increased by almost 3 percentage points each decade for those born between 1962 to 1975 - but then saw a steady decline among those born after 1975. Studies in Denmark, Britain, France, the Netherlands, Finland and Estonia have demonstrated a similar downward trend in IQ scores. "The causes in IQ increases over time and now the decline is due to environmental factors," said [study co-author Ole] Rogeburg. These environmental factors could include changes in the education system and media environment, nutrition, reading less and being online more, Rogeberg said. Researchers have long preferred to use genes to explain variations in intelligence over environmental factors. However, the new study turns this thinking on its head. Intelligence is heritable, and for a long time, researchers assumed that people with high IQ scores would have kids who also scored above average. Moreover, it was thought that people with lower scores would have more kids than people with high IQ scores. The study not only showed IQ variance between children the same parents, but because the authors had the IQ scores of various parents, it demonstrated that parents with higher IQs tended to have more kids, ruling out the dysgenic fertility theory as a driver of falling IQ scores.