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Researchers have known for years that first-born children are more likely to end up better educated and better paid than their later-born siblings.

Now, thanks to a recent study by Brigham Young University economics professor Joseph Price published in Journal of Human Resources, there's a clearer explanation of why that's so.

First-born children are also first in line to claim the majority of their parents' time. And time spent with children, it's well known, is crucial to their early development and subsequent growth into successful, thriving adults.

It's not that parents spend less time with their younger children on purpose. Rather, parents tend to spend less quality time with their children as the family ages, Price found.

Parents wishing to treat all their children equally have two choices, he said. They can either space the birth of their children closer together in time, or make a conscious effort not to lose stamina while raising their children over the years.

"If you're at that stage when your kids are a little older, your inclination is to watch a movie with them, but that's also a disservice to your younger children," Price said. "But when your first-born child was 4 you probably spent time reading to that child or playing or talking. This should remind us of the fact of how important we are as parents."

Price warns that parents who think they can escape this tendency by limiting themselves to just one child are mistaken. Only children ages 4 to 5 end up spending less time with their parents, on average between 90 minutes per day with the father and 116 minutes per day with the mother, than do parents with two children. In that situation the father spends 115 minutes per day on average with his children, while the mother spends an average of 177 minutes per day with her two children.

The research is significant because it lends an understanding to what causes birth order differences, Sandra Black, co-editor of Journal of Human Resources said in a statement. Price used data from the American Time Use Survey, a federal government study involving 21,000 people.

"It's among the first to use a large data set to document systematic differences in parental investments by birth order," Black said.

Price himself is first-born among five siblings. It's far from clear who's ahead in terms of income or professional success, however. Identical to Price, the next oldest of his brothers is earning a doctorate in economics at Cornell University. Another brother is earning his doctorate in political science at Purdue University. "It's kind of a dead tie," Price said.

* First-born children get about 3,000 more hours of quality time with their parents between ages 4 and 13 than the next sibling gets when they pass through the same age range.

* Not only do parents spend less total time with children as the family ages, but more of that time is spent on activities not considered to be "quality" time, such as watching TV.

* The youngest child gets about the same amount of quality time whether the family is large or small. Parents of large families devote more overall quality time to their children, so the youngest of four siblings ends up with as much quality time as the younger of two siblings.