313 pairs of brothers and 296 pairs of sisters were compared by one or both parents in a postal questionnaire. For 495 pairs independent questionnaires were completed by father and mother, and agreement between parents was good.

There was a marked tendency for the first-born child to be rated as less fond of cuddling; he was also easier to train, worked harder at school, set himself higher standards, and was more serious, methodical, law-abiding, tidy and less impulsive; he learned to talk and read at a younger age and was rated as having more natural ability at schoolwork. These three groups of first-born attributes were relatively independent; for instance, even the first-born with less ability at schoolwork were given more responsibility at school. In the 152 pairs which did not contain a first-born, little birth order effect could be found, whether or not the elder was the eldest of his or her sex.

There was a slight tendency for the broader, fatter and more muscular child to be less nervous and highly strung, but otherwise differences in physique were not related to differences in behaviour. Abilities at schoolwork, games, music and painting were not related. Tendency to anxiety and depression was related to lack of sociability; to stubbornness and lack of practicality; to angryness but not the bottling up of anger; to level of aspiration but not the possession of abilities or good looks. These relations of anxiety and depression in a normal sample closely parallel the items which distinguish psychiatric patients from their healthy siblings.