title: Effect of household size on mental problems in children: results from the Norwegian Mother and Child Cohort study.
abstract: BACKGROUND
  Most people in industrialized societies grow up in core (parents only) families with few if any siblings. Based on an evolutionary perspective, it may be argued that this environment reflects a mismatch, in that the tribal setting offered a larger number of close affiliates. The present project examined whether this mismatch may have a negative impact on mental health.
  
  
  METHODS
  We used data from the Norwegian Mother and Child Cohort Study (MoBa), which includes 114 500 children. The mothers were recruited during pregnancy and followed up with questionnaires as the infants grew older. Correlates between number and type of people living in the household and questions probing mental health were corrected for likely confounders.
  
  
  RESULTS
  The number of household members correlated with scores on good mental health at all ages tested (3, 5 and 8 years). The effects were distinct, highly significant, and present regardless of how mental issues were scored. The outcome could be attributed to having older siblings, rather than adults beyond parents. The more siblings, and the closer in age, the more pronounced was the effect. Living with a single mother did not make any difference compared to two parents. Girls were slightly more responsive to the presence of siblings than boys. Household pets did not have any appreciable impact.
  
  
  CONCLUSIONS
  A large household is associated with fewer mental problems in children.
authors.affiliation: Division of Mental Health, Norwegian Institute of Public Health, Postbox 4404, Nydalen, 0403, Oslo, Norway. bjgr@fhi.no.
authors.forename: Bjørn
authors.initials: B
authors.lastname: Grinde
authors.affiliation: Division of Mental Health, Norwegian Institute of Public Health, Postbox 4404, Nydalen, 0403, Oslo, Norway.
authors.forename: Kristian
authors.initials: K
authors.lastname: Tambs
mesh_terms: D000328:Adult; D005075:Biological Evolution; D002648:Child; D002653:Child Behavior Disorders / Q000453:epidemiology*; D005191:Family Characteristics; D005260:Female; D006801:Humans; D008297:Male; D001523:Mental Disorders / Q000453:epidemiology*; D008603:Mental Health / Q000706:statistics & numerical data*; D009034:Mother-Child Relations; D009035:Mothers; D009664:Norway; D011446:Prospective Studies; D035781:Siblings; D012919:Social Behavior; D055815:Young Adult
keywords: Birth order; Childhood; Evolutionary perspective; Household size; Mental problems; MoBa; Siblings; Social affiliations