Child birth order study

Oldest children get the lion's share of parental quality time
First-born children may receive an average of 3,000 more hours of quality time with their parents compared to subsequent children, according to a new study seeking to explain the differences in achievement between siblings. Researchers say the discrepancy is not because parents don't try to divide their time equally, but because first-time parents often are able to show their oldest child more attention before younger siblings are born and child-rearing becomes more chaotic.
Price, J. (2008). Parent-child quality time. Journal of Human Resources, XLIII: 240 - 265.