Does birth order affect temperament?

Many parents wonder whether their child is demanding or fearful or has some other temperament characteristics... simply because the child was first born. Or the last baby. Or in between. Were parents too anxious, too ready to give attention to their first child? Too slow to allow independence to their final baby? Too busy with the first and last child to pay enough attention to the middle one?

In our Kaiser Permanente studies, we found no link between birth order and parents' perception of their infant's temperament at four months. But birth order did play a role in determining the strength of the relationship between the child's temperament and the occurrence of later behavioral issues.

For example, if an infant was easily frustrated and intense in reactions at four months, separation issues later on were more likely if the infant was also first born. So birth order doesn't affect temperament, but the consequences of temperament, operating through the goodness-of-fit between the child's initial temperament and the parenting style generated by the infant's birth order.

There are various reasons why parents of first or last-born infants may adopt an over-accommodating style. The inexperience of parents of first-borns may make them more anxious and willing to get up repeatedly at night. Or the fear of loss of the parenting role may make parents of the "last baby" more willing to bring that toddler into bed with them. But with respect to future behavioral problems, these possibilities don't seem to make a difference... unless the child's temperament is energetic or slow adapting or low in frustration tolerance.