Of course parents are concerned about their children's health, but the aspect of birth order that parents probably ponder most is behavior. Parents who have more than one child may set their expectations based on their experiences with a previous child, only to find that another child breaks the mold.
Published research from Norway proposes that firstborns are often the most intelligent. One possible reason for this is "mentoring younger siblings and helping with tasks provides the IQ boost."
A Harvard study found that children are more likely to be friends with people from the same birth-order group: "The authors say certain personality traits—like dominance among firstborns, or individuality among youngest—may dictate the type of people you gravitate toward, though these personality traits have not yet been proved."
It sounds cliché, but studies have shown that older children tend to be people-pleasers and more achievement oriented while younger children are more likely to rebel. A firstborn child gets lots of attention while children that are born later must share. While it is impossible for parents to divide time and all resources equally, they can make an effort to give each child some undivided attention.