Editing Twin study (section)

==== Measured similarity: A direct test of assumptions in twin designs ====
A particularly powerful technique for testing the twin method was reported by Visscher ''et al.''<ref>{{Cite journal | last1 = Visscher | first1 = Peter M.| last2 = Medland | first2 = Sarah E.| last3 = Ferreira | first3 = Manuel A. R.| last4 = Morley | first4 = Katherine I.| last5 = Zhu | first5 = Gu| last6 = Cornes | first6 = Belinda K.| last7 = Montgomery | first7 = Grant W.| last8 = Martin | first8 = Nicholas G.| doi = 10.1371/journal.pgen.0020041 | title = Assumption-Free Estimation of Heritability from Genome-Wide Identity-by-Descent Sharing between Full Siblings | journal = [[PLOS Genetics]] | volume = 2 | issue = 3 | pages = e41 | year = 2006 | pmid =  16565746| pmc =1413498 | doi-access = free}}</ref> Instead of using twins, this group took advantage of the fact that while siblings on average share 50% of their genes, the actual gene-sharing for individual sibling pairs varies around this value, essentially creating a continuum of genetic similarity or "twinness" within families. Estimates of heritability based on direct estimates of gene sharing confirm those from the twin method, providing support for the assumptions of the method.