First-born children have always known it's tough being the eldest, and now a study of birth order and stress in rhesus monkeys has confirmed it.

In stressful situations, first-born infant monkeys produce up to twice as much of the stress hormone cortisol as their younger siblings -- and mother might be to blame, scientists announced at the fourth annual Wisconsin Symposium on Emotion in Madison.

Psychologists Steven Shelton, Ned Kalin and colleagues at the University of Wisconsin in Madison exposed 13 female and 15 male monkeys aged 7 1/2 months to fear-inducing situations, such as an unfamiliar human entering the room when the monkey was alone.

On average, first-born monkeys had significantly higher levels of cortisol, and some had as much as twice the levels measured in their later-born siblings. The monkeys with high cortisol levels would also freeze in one spot for up to four times as long as the others.

Pub Date: 5/04/98