Editing Twin study (section)

=== Assumptions ===

It can be seen from the modeling above, the main assumption of the twin study is that of equal family environments, also known as the '''equal environments assumption'''.<ref>{{Cite journal |title=The equal environments assumption of classical twin studies may not hold |journal = British Journal of Educational Psychology|volume = 75|issue = 3|last1=Richardson |first1=Ken |last2=Norgate |first2=Sarah |date=2005-09-01 |pages=339–350 |language=en |doi=10.1348/000709904x24690 |pmid = 16238870|url = http://usir.salford.ac.uk/93/5/EP200506revised.pdf}}</ref><ref>{{Cite journal |last1=Plomin |first1=Robert |last2=Willerman |first2=Lee |last3=Loehlin |first3=John C. |date=1976-03-01 |title=Resemblance in appearance and the equal environments assumption in twin studies of personality traits |journal=Behavior Genetics |language=en |volume=6 |issue=1 |pages=43–52 |doi=10.1007/bf01065677 |pmid=943160 |s2cid=30732913 |issn=0001-8244}}</ref><ref>{{Cite web |url=http://www.apa.org/monitor/apr04/second.aspx |title=Behavioral Genetics--A second look at twin studies |last=Winerman |first=Lea |date=2004-04-01 |website=Monitor on Psychology |language=en |access-date=2017-08-23}}</ref> A special ability to test this assumption occurs where parents believe their twins to be non-identical when in fact they are genetically identical. Studies of a range of psychological traits indicate that these children remain as concordant as MZ twins raised by parents who treated them as identical.<ref>{{cite journal | last1 = Kendler | first1 = K.S. | last2 = Neale | first2 = M.C. | last3 = Kessler | first3 = R.C. | last4 = Heath | first4 = A.C. | last5 = Eaves | first5 = L.J. | year = 1993 | title = Panic disorder in women: A population-based twin study | journal = Psychological Medicine | volume = 23 | issue = 2| pages = 397–406 | doi=10.1017/s003329170002849x| pmid = 8332656 | s2cid = 30324607 }}</ref>

Molecular genetic methods of heritability estimation have tended to produce lower estimates than classical twin studies due to modern SNP arrays not capturing the influence of certain types of variants (e.g., rare variants or repeat polymorphsisms), though some have suggested it is because twin studies overestimate heritability.<ref>{{Cite journal |last=Felson |first=Jacob |date=January 2014 |title=What can we learn from twin studies? A comprehensive evaluation of the equal environments assumption |journal=[[Social Science Research]] |volume=43 |pages=184–199 |doi=10.1016/j.ssresearch.2013.10.004 |pmid=24267761 |issn=0049-089X|quote=...estimates of cumulative genetic influence using molecular-level data have tended to be substantially lower than the corresponding estimates from twin studies.}}</ref> A 2016 study determined that the assumption that the prenatal environment of twins was equal was largely tenable.<ref>{{cite journal|last1=Van Beijsterveldt|first1=C. E. M.|last2=Overbeek|first2=L. I. H.|last3=Rozendaal|first3=L.|last4=McMaster|first4=M. T. B.|last5=Glasner|first5=T. J. |last6=Bartels|first6=M. |last7=Vink|first7=J. M. |last8=Martin|first8= N. G. |last9=Dolan|first9=C. V.| last10=Boomsma|first10=D. I.|title=Chorionicity and heritability estimates from twin studies: The prenatal environment of twins and their resemblance across a large number of traits.|journal=Behavior Genetics|volume=46|number=3|year=2016|pages=304–314|doi=10.1007/s10519-015-9745-3|pmid=26410687|pmc=4858554}}.</ref> Researchers continue to debate whether or not the equal environment assumption is valid.<ref>{{cite journal | last1 = Conley | first1 = Dalton | last2 = Rauscher | first2 = Emily | last3 = Dawes | first3 = Christopher | last4 = Magnusson | first4 = Patrik KE | last5 = Siegal | first5 = Mark L. | year = 2013 | title = Heritability and the equal environments assumption: Evidence from multiple samples of misclassified twins | journal = Behavior Genetics | volume = 43 | issue = 5| pages = 415–426 | doi=10.1007/s10519-013-9602-1| pmid = 23903437 | hdl = 1808/24764 | s2cid = 2083213 | hdl-access = free }}</ref><ref>Fosse, Roar, Jay Joseph, and Ken Richardson. "A critical assessment of the equal-environment assumption of the twin method for schizophrenia." Frontiers in psychiatry 6 (2015): 62.</ref><ref>{{cite journal | last1 = Barnes | first1 = J. C. | last2 = Paul Wright | first2 = John | last3 = Boutwell | first3 = Brian B. | last4 = Schwartz | first4 = Joseph A. | last5 = Connolly | first5 = Eric J. | last6 = Nedelec | first6 = Joseph L. | last7 = Beaver | first7 = Kevin M. | year = 2014 | title = Demonstrating the validity of twin research in criminology | journal = Criminology | volume = 52 | issue = 4| pages = 588–626 | doi=10.1111/1745-9125.12049}}</ref><ref>Joseph, Jay. The trouble with twin studies: A reassessment of twin research in the social and behavioral sciences. Routledge, 2014.</ref><ref>{{cite journal | last1 = Felson | first1 = Jacob | year = 2014 | title = What can we learn from twin studies? A comprehensive evaluation of the equal environments assumption | journal = Social Science Research | volume = 43 | pages = 184–199 | doi=10.1016/j.ssresearch.2013.10.004 | pmid=24267761}}</ref>

==== 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.