title: Is low mood an adaptation? Evidence for subtypes with symptoms that match precipitants.
abstract: BACKGROUND
  Although severe depression is dysfunctional, the capacity to experience normal low mood may be useful in certain fitness-threatening situations. Moreover, if specific kinds of situations recurred often enough in the course of evolution, natural selection may have shaped partially differentiated subtypes of low mood that are parallel to the subtypes of anxiety that protect against different kinds of danger. To test this hypothesis, we examined how symptoms of low mood differ depending upon the precipitating situation, and whether these differences match expectations of symptoms useful in each kind of situation.
  
  
  METHODS
  337 subjects who experienced a period of low mood within the last year wrote accounts describing perceived causes of their low mood and they filled out the CES-D depression inventory. Seven symptom scales were derived from analysis of CES-D data. Independent judges blindly coded the accounts into one of six precipitant categories.
  
  
  RESULTS
  Different untoward situations were associated with different symptoms that were predicted to be useful in those situations. Social losses (death of a loved one, romantic breakups, and social isolation) were associated with greater crying and arousal. Failure to reach a goal, stress, and winter seasons were associated with more fatigue and pessimism.
  
  
  CONCLUSIONS
  These results suggest that natural selection shaped not only a generic state of low mood but also partially differentiated subtypes shaped to cope with specific situations that were associated with fitness losses among our ancestors.
authors.affiliation: Center for Society and Genetics (UCLA), Box 951405, 1339 Murphy Hall, Los Angeles, CA, 90095, USA. matthew.c.keller@gmail.com
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Keller
authors.forename: Randolph M
authors.initials: RM
authors.lastname: Nesse
mesh_terms: D000223:Adaptation, Psychological; D000328:Adult; D000339:Affect; D003866:Depressive Disorder / Q000523:psychology*; D005260:Female; D006801:Humans; D007398:Interpersonal Relations; D008016:Life Change Events; D008297:Male; D012307:Risk Factors; D012934:Social Isolation