Background: It is important to detect cognitive decline at an early stage, especially before onset of mild cognitive impairment and dementia. Processing speed and working memory are aspects of cognitive function that are associated with cognitive decline. Hand strength is an inexpensive, easily measurable indicator of cognitive decline. However, associations between hand strength, processing speed, and working memory have not been studied. In addition, the genetic and environmental structure of the association between hand strength and cognitive decline is unclear. We investigated phenotypic associations between hand strength, processing speed, and working memory and examined the genetic and environmental structure of the associations between phenotypes. Methods: Hand strength, processing speed (digit symbol performance), and working memory (digit span performance) were examined in monozygotic and dizygotic twin pairs. Generalized estimating equations were used to identify phenotypic associations, and structural equation modeling was used to investigate the genetic and environmental structure of the association. Results: Generalized estimating equations showed that hand strength was phenotypically associated with digit symbol performance but not with digit span performance. Structural equation modeling showed that common genetic factors influenced hand strength and digit symbol and digit span performance. Conclusions: There was a phenotypic association between hand strength and processing speed. In addition, some genetic factors were common to hand strength, processing speed, and working memory.
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