"There is a long history of research suggesting that firstborns tend to have a slightly higher IQs than later-born children, but psychologists have disagreed about these findings. Researchers have finally established a negative relationship between birth order and intelligence, meaning that later-born siblings, on average, have slightly lower IQs than firstborns (Kristensen & Bjerkedal, 2007). The data suggest that environment is responsible for this effect."

Psychological Science.