Researchers have found interesting facts about birth order. Psychologists say many firstborns face pressure to succeed from their parents. They tend to be the parents' main focus until a brother or sister is born. These facts may help explain why firstborn children tend to be high achieving. Firstborns tend to score higher on intelligence tests and are usually taller and healthier than siblings born after them. Firstborns tend to be highly successful. More than half of all members of Congress are firstborns, as have been many U.S. presidents. Studies show middle children can feel lost in a family because they have to deal with parents on the one hand and with siblings on the other. This position often helps them learn to negotiate conflicts. It can help make them highly competitive. They are often less self-confident than their siblings and are the most likely to get into arguments and become rebels. Researchers has shown that youngest children can be the focus of the rest of the family, but sometimes feel lost while the focus is on older children. They tend to be the most emotionally sensitive siblings. That may be why many of them draw attention to themselves with humor or artistic interests. This fact may help explains why many youngest children become artists or performers. Not everyone, however, agrees that birth order is important. For example, Serena Williams says she is motivated by sibling rivalry, not birth order. "No one gets more motivated than [Venus], her sister, because I don't want her to catch up with me."