One of the problems with too few smart, successful people having children and too many poor, unsuccessful people having children has been an increase in the percentage of premature births (women from low-socio-economic backgrounds are more likely to have pre-mature births).
However, after trending upwards for several decades, the percentage of pre-mature births (in the US at least) has peaked and is now starting to decline slightly.
Premature infants are more likely to suffer from a range of physical and mental health problems including low intelligence, Autism, ADHD, heart defects, and diabetes.
Commonwealth Contrarian 