Your family—the number of siblings you have and how old they are—has a big effect on whom you become, research suggests. For one thing, there's intellect: a large Norwegian study just confirmed that first-borns have slightly higher IQs than their younger siblings do. Because the study found that second-born children whose older sibling died at a young age are also slightly smarter and because “only” children do not show this IQ advantage, the intellectual disparity is more likely to be the result of differences in a child's environment after birth than of biological effects. Scientists speculate that eldest kids communicate with and coach their younger siblings, which requires them to consolidate knowledge at a young age and potentially gives them a slight intellectual edge. These findings build upon a body of research suggesting birth order and family size influence a number of traits and risk factors.