title: Is There a Female Protective Effect Against Attention-Deficit/Hyperactivity Disorder? Evidence From Two Representative Twin Samples.
abstract: Attention-deficit/hyperactivity disorder (ADHD) is more frequent in males than in females. The "female protective effect" posits that females undergo greater exposure to etiological factors than males in order to develop ADHD, leading to the prediction that relatives of females with ADHD will display more ADHD behaviors. We thus tested whether cotwins of females displaying extreme ADHD traits would display more ADHD traits than cotwins of males displaying extreme ADHD traits.
  
  
  
  Parents of approximately 7,000 pairs of nonidentical twins in Sweden, and approximately 4,000 pairs of twins in England and Wales, completed dimensional assessments of ADHD traits. Probands were selected on the basis of scoring within the highest 10% of the distribution in each sample. Dimensional scores of cotwins of probands, as well as the categorical recurrence rate, were investigated by proband sex.
  
  
  
  Cotwins of female probands displayed higher mean ADHD trait scores (mean = 0.62-0.79) than cotwins of male probands (mean = 0.38-0.55) in both samples. This trend was significant in the Swedish sample (p < .01) and when the 2 samples were merged into a single, larger sample (p < .001). When the samples were merged, there was also a significant association between proband sex and cotwin's categorical status, with more cotwins of female probands also being probands than cotwins of male probands.
  
  
  
  These findings support a female protective effect against ADHD behaviors, suggesting that females require greater exposure to genetic and environmental factors associated with ADHD in order to develop the condition.
authors.affiliation: Karolinska Institutet, Stockholm.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Taylor
authors.affiliation: Karolinska Institutet, Stockholm.
authors.forename: Paul
authors.initials: P
authors.lastname: Lichtenstein
authors.affiliation: Karolinska Institutet, Stockholm.
authors.forename: Henrik
authors.initials: H
authors.lastname: Larsson
authors.affiliation: Centre for Ethics, Law and Mental Health (CELAM), University of Gothenburg, Gothenburg, Sweden.
authors.forename: Henrik
authors.initials: H
authors.lastname: Anckarsäter
authors.affiliation: Radboud University Medical Center, Donders Institute for Brain, Cognition and Behaviour, Nijmegen, The Netherlands; Karakter Child and Adolescent Psychiatry Center, Nijmegen; and King's College London, Medical Research Council Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, Psychology and Neuroscience, London.
authors.forename: Corina U
authors.initials: CU
authors.lastname: Greven
authors.affiliation: Centre for Brain and Cognitive Development, Birkbeck, University of London. Electronic address: a.ronald@bbk.ac.uk.
authors.forename: Angelica
authors.initials: A
authors.lastname: Ronald
mesh_terms: D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology*; D002648:Child; D004200:Diseases in Twins; D004739:England / Q000453:epidemiology; D005260:Female; D006801:Humans; D008297:Male; D065840:Protective Factors; D012737:Sex Factors; D013548:Sweden / Q000453:epidemiology; D014852:Wales / Q000453:epidemiology
keywords: ADHD; genetics; sex differences; twin study