Theories abound about the effects of birth order on personality, success, and other life outcomes. Last week we quizzed our readers on traits that differed between 23andMe customers who were oldest children (firstborns) versus youngest children (lastborns). Congratulations to Kay, the winner of the quiz! Kay got all three answers correct and will receive a $25 Amazon gift card. So what were the correct answers? Read on… Some researchers have theorized that firstborns are more successful than younger children (lastborns). Though they don’t have a leg up on all potential measures of success, 23andMe’s firstborns are more educated than lastborns. Even controlling for age, sex, and correcting for multiple tests (as we did for all the findings reported here), firstborns reported greater educational attainment than lastborns. We can’t say that this fully confirms the idea that firstborns are more successful, as they did not report higher incomes or higher subjective socioeconomic status.Personality theorists have also postulated that firstborns are more conscientious, more socially dominant, less agreeable, and less open to new ideas than lastborns. In actuality, we found that firstborns are more full of ideas, but less altruistic than lastborns. These results are consistent with some but not all of the existing, but controversial, research on firstborn children’s personalities. While altruism is a component of agreeableness, and therefore might seem to support the theory, firstborns did not differ significantly in overall agreeableness, just altruism. Why might this be? We’re just not sure. Perhaps lastborns have to learn to share from an earlier age than do firstborns. Have a theory? Leave it in the comments below.Finally, firstborns are more likely to have had chicken pox or mumps. This was really interesting and completely unexpected. It’s possible that having an older sibling might reduce your chance of contracting certain childhood diseases.As an aside, one of the red herrings for this last question was that firstborns are more likely to smoke. In fact, the opposite was true. Lastborns were significantly more likely to have smoked than firstborns. About a third of lastborns had smoked regularly at some point in their life, compared to roughly a quarter of firstborns.Have your own theories about birth order effects? Leave a comment below!