The first-born is likely to be the smartest, according to research First-born children are more intelligent than their siblings, research has found. The oldest child is more likely to have a higher IQ, and the youngest likely to have the lowest, say researchers. Scientists at Vrije University, Amsterdam, studied 650 children whose IQs were tested three times until they were 18. Dorret Boomsma, whose work will be published in the journal Intelligence, said: "The effect is seen in boys and girls. "The highest IQ scores were in children without any older siblings, followed by children with one older sibling. "Children with two or more older siblings obtained the lowest score." Why this occurs is unclear but it is thought that the level of attention parents lavish on first-borns boosts their intellectual development. But while the oldest may be the brightest, another study shows the youngest is the most favored. Research on 2,000 families by the University of California found first-borns are achievers, who are dominant, religious, conscientious and neurotic. Middle-borns are rebellious, less religious, impulsive and open to new experiences. Last-borns are agreeable, sociable and creative - and are often the most favored child.