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Unintentional injuries (accidents) are a leading cause of premature mortality, morbidity and disability. 1 In people under 45 years of age, injuries are more common than chronic diseases, representing a major economic burden because of increased healthcare costs. In spite of the public health importance, little is known about risk factors for unintentional injury. However, a small number of studies have recently identified low childhood cognitive function/intelligence (denoted IQ) as a risk factor for subsequent non-fatal injury in adulthood. 2-4 Some studies have also reported inverse associations between IQ and mortality from all unintentional injuries, 5 6 road traffic accidents, 5 7 and accidental falls, poisoning, fire and drowning. 5
Many of the existing studies have offered only modest statistical power and limited insights into the relationships between IQ and the full range of specific injury subtypes. In the present study, we explore associations between IQ measured in early adulthood and subsequent hospital admissions for unintentional injury in a cohort of more than 1 million Swedish men, of whom almost 200 000 had at least one hospitalisation for unintentional injury.
Methods
Study participants and record linkage
The record linkage used to generate this cohort has been reported previously. 8 9 The cohort comprised all non-adopted men born in Sweden from 1950 to 1976 with both biological parents identified in the Multi-Generation Register. Using unique personal identification numbers, we linked this register with the housing census records (1960 and 1970) and Military Service Conscription, Population and Cause of Death and National Hospital Discharge Registers, resulting in 1 379 531 successful matches. Study approval was obtained from the Regional Ethics Committee, Stockholm.
Conscription examination
The military service conscription examination involves structured, standard medical assessment of physical and mental health, and cognitive function. During the years covered by this study, the law required this examination; only men of foreign citizenship or with severe disability were excused. This dataset covers examinations from 15 September 1969 to 31 December 1994, after which IQ measurement procedures changed. Average age at examination was 18.3 years (range 16-25).
IQ was measured by four written subtests representing verbal, logical, spatial and technical abilities. 10 The verbal test measures knowledge of synonyms. The logical test measures the capacity to understand written instructions and apply them...