For the first time, a 2008 study has produced empirical evidence about the possible causes. First-born children receive 20 to 30 more minutes of daily quality time from parents than second-born kids of the same age from similar families, says the research. Although many people have hypothesized that firstborns get more parental time, this study, by Joseph Price of Brigham Young University, is the first I’ve seen that has the data to prove it.

Most other studies have focused on outcomes – on children’s personalities and traits later in life – and have found firstborns tend to have higher IQs and to be more high-achieving and conscientious; later-born children tend to be more rebellious, liberal and agreeable.