When IQ tests are taken by each generation, they are always normalized so that the average is a score of 100. However, when people are asked to take tests from older generations, they tend to do better than the older generations did. This is the Flynn effect. It was very consistent throughout the second half of the 20th century and into the 21st, but a number of studies in Scandinavia indicate that it may have reversed more recently. Immigration is one controversial explanation that has been suggested, whether due to genetic or cultural reasons. But the study I link to here tested the effect within families against the effect across unrelated people, and found that it is just as strong within families. This suggests environmental factors such as education and nutrition as the culprits, although the article does still put “migration” in this category. I suppose if you had a society with a high quality of nutrition and education, and you then have an influx of new people with more bad habits (let’s say, a high rate of smoking), that could have an effect.