Parent-Child Quality Time
利用美国时间使用调查数据，发现头胎孩子每天比二胎孩子多获得20-30分钟的亲子优质时间，且这种差异源于父母同时给予每个孩子大致相同的时间，而亲子优质时间随孩子年龄增长而减少。
Using data from the American Time Use Survey, I find that a first-born child receives 20-30 more minutes of quality time each day with his or her parent than a second-born child of the same age from a similar family. The birth-order difference results from parents giving roughly equal time to each child at any point in time while the amount of parent-child quality time decreases as children get older. These results provide a plausible explanation for recent research showing a very significant effect of birth order on child outcomes.