A congenital factor in behaviour disturbance, including those types thereof related to delinquency, was inferred from data drawn from two sources.A recomputation was made of the delinquency proneness of boys by year of birth, as published by Wilkins. This showed that when limited to those committing offences between their 8th and 14th birthdays, the proneness was highly specific to those boys born during the early war years. This wartime peak closely resembled that for children dying from congenital malformation in the first 4 weeks of life. There was no such tendency among those commtting offences after their 14th birthday.Among Glasgow boys put on probation in 1957 and non-delinquent controls, a close relationship was found between a number of physical conditions—respiratory and other ailments, physical defects, bad eyesight and abnormality of growth—and indications of behaviour disturbance as recorded on the Bristol Social Adjustment Guides. With the exception of bad eyesight among the controls, the incidence of these conditions was never less than twice as high among the maladjusted compared with stable group; and among probationers and controls combined the difference was signficant for all 5 conditions at a risk of chance of less than 1 in a 1000.Three possible alternatives were considered to the hypothesis of a congenital factor common to the physical conditions and the behaviour disturbance. Inadequate living standards could not be the explanation since the associations held good among those probationers coming from adequate standard homes. Compensation for feelings of inferiority or frustration was similarly ruled out, because boys showing this type of behaviour disturbance had lower than average physical illness or defect. Finally a psychosomatic explanation—that the maladjustment produced exhaustion which undermined health—was rejected, first because the associations held good when the depressed children were excluded, and second because it could not feasibly account for the defects as distinct from the illnesses.It was consequently argued that the only explanation known which could account for the associations was that of congenital insult which in some cases was seen both somatically and in impairment of that part of the nervous system controlling behaviour. The latter might induce a greater delinquency proneness by reducing resistance to stress, and thus more frequent breakdown under adverse environmental conditions.Abstract Teaser