Determinants of adult psychomotor speed : a study of monozygotic twins

Abstract

The determinants of hand and foot psychomotor speed were investigated in monozygotic twins 35 to 69 years of age. First, the reliability study was conducted to investigate the consistency of the reaction time measurements using the method selected. Second, the effects of exercise, smoking and driving were examined by contrasting co-twins discordant for the factor studied. Finally, the contribution of these factors and others suspected of influencing psychomotor speed were studied using a multivariate model. Recruited from the population-based Finnish Twin Cohort, including 2,050 male monozygotic twin pairs, were 61 relatively healthy twin pairs. Psychomotor speed was determined by simple and choice visual signal tests using the hand and foot, yielding measurement of decision time, movement time, and their sum, reaction time. The consistency of the within-test and between-test sessions was acceptable (r=.49-.99) for both hand and foot measurements. Twins with a history of strenuous and frequent exercise had faster psychomotor speed than their co-twins with histories of moderate, occasional lifetime exercise. Twins with a history of smoking, as well as twins with histories of excessive vehicular driving tended to have slower psychomotor speed than their less exposed co-twins. The major determinant of psychomotor speed, however, was familial aggregation, a combination of genetic and shared early environmental influences, which explained 18-52% of the variation in psychomotor speed, depending on the extremity and psychomotor speed outcome measured. Other determinants accounted for 0- 17% of the variation, with age alone explaining up to 13% of the variation in psychomotor speed. Other factors which had more minor effects were cardiovascular morbidity, associated with impaired psychomotor speed, and strenuous exercise and sedentary work which were associated with slightly faster speeds. These results suggest that promoting a healthy lifestyle that includes vigorous exercise and minimal cardiovascular disease may have a modest effect on reducing the age-related decline in psychomotor speed. A combination of genetics and family environment influences appear to have considerable effect on psychomotor speed in later adulthood, yet much of the variability in psychomotor speed remains unexplained.