Larger family size decreases childhood cognitive abilities and increases behavioral problems

This study estimates the impact of increases in family size on childhood and adult outcomes using matched mother-child data from the National Longitudinal Survey of Youth 1979.

The study finds that families face a substantial quantity-quality trade-off:  increases in
family size decrease parental investment, decrease childhood cognitive abilities, and increase behavioral problems.  

The negative effects on cognitive abilities are much larger for girls while the detrimental effects on behavior are larger for boys.  The study also finds evidence of heterogeneous effects by mother's AFQT score, with the negative effects on cognitive scores being much larger for children of mothers with low AFQT scores.