title: Methodological issues in twin research: the assumption of environmental equivalence.
mesh_terms: D002657:Child Development; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D007744:Obstetric Labor Complications; D011247:Pregnancy; D011248:Pregnancy Complications; D012107:Research Design; D012737:Sex Factors; D012923:Social Class; D012931:Social Environment; D011795:Surveys and Questionnaires; D014427:Twins; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic