title: Explaining the sex difference in depression with a unified bargaining model of anger and depression.
abstract: BACKGROUND
  Women are twice as likely as men to be depressed, a bias that is poorly understood. One evolutionary model proposes that depression is a bargaining strategy to compel reluctant social partners to provide more help in the wake of adversity. An evolutionary model of anger proposes that high upper body strength predisposes individuals to angrily threaten social partners who offer too few benefits or impose too many costs. Here, we propose that when social partners provide too few benefits or impose too many costs, the physically strong become overtly angry and the physically weak become depressed. The sexual dimorphism in upper body strength means that men will be more likely to bargain with anger and physical threats and women with depression.
  
  
  METHODS
  We tested this idea using the 2011-12 National Health and Nutrition Examination Survey (NHANES), a large nationally representative sample of US households that included measures of depression and upper body strength.
  
  
  RESULTS
  A 2 SD increase in grip strength decreased the odds of depression by more than half ([Formula: see text],[Formula: see text]), which did not appear to be a consequence of confounds with anthropometric, hormonal or socioeconomic variables, but was partially explained by a confound with physical disability. Nevertheless, upper body strength mediated 63% of the effect of sex on depression, but the mediation effect was unexpectedly moderated by age.
  
  
  CONCLUSIONS
  Low upper body strength is a risk factor for depression, especially in older adults, and the sex difference in body strength appears to explain much of the perplexing sex difference in depression.
authors.affiliation: Department of Anthropology, Washington State University, 14204 NE Salmon Creek Avenue, Vancouver, WA 98686, USA edhagen@wsu.edu.
authors.forename: Edward H
authors.initials: EH
authors.lastname: Hagen
authors.affiliation: Institute of Behavioural Sciences, University of Helsinki, Helsinki 00014 (PO Box 9), Finland.
authors.forename: Tom
authors.initials: T
authors.lastname: Rosenström
keywords: bargaining; depression; grip strength; sex difference