vining's research has indicated that the average iq of americans has decreased by about 5 points since iq tests became widely used in the early 20th century, and that high-intelligence people tend to have as many or more children as people who are less intelligent during periods when the overall fertility rate is rising. his research has also found that wealthy families tend to have as many, or fewer, children than do low-income families; he has argued that this pattern occurs because human reproductive behavior is learned, not heritable.