Editing Twin study (section)

== Criticism ==
The twin method has been subject to criticism from statistical [[genetics]], statistics and [[psychology]], with some researchers, such as Burt & Simons (2014), arguing that conclusions reached via this method are ambiguous or meaningless.<ref name=burtsimons>{{cite journal|last1=Burt|first1=Callie|last2=Simons|first2=Ronald L.|author-link2=Ronald L. Simons|title=Pulling back the curtain on heritability studies : Biosocial criminology in the postgenomic era|journal=[[Criminology (journal)|Criminology]]|date=May 2014|volume=52|issue=2|pages=223–262|doi=10.1111/1745-9125.12036}}</ref> Core elements of these criticisms and their rejoinders are listed below.

=== Criticisms of fundamental assumptions ===
Critics of twin studies argue that they are based on false or questionable assumptions, including that monozygotic twins share 100% of their genes<ref>{{Cite book |title=Embodiment and Epigenesis: Theoretical and Methodological Issues in Understanding the Role of Biology within the Relational Developmental System – Part B: Ontogenetic Dimensions |volume=45 |last=Ho |first=Mae-Wan|author-link=Mae-Wan Ho |date=2013 |publisher=[[Elsevier]] |isbn=9780123979469 |editor-last=Lerner |editor-first=Richard |editor-link=Richard M. Lerner |pages=67–92 |chapter=No Genes for Intelligence in the Fluid Genome |doi=10.1016/b978-0-12-397946-9.00004-x |pmid=23865113 |editor-last2=Benson |editor-first2=Janette B.|series=Advances in Child Development and Behavior }}</ref> and the equal environments assumption.<ref>{{Cite journal |last=Joseph |first=Jay |author-link=Jay Joseph|date=2002 |title=Twin Studies in Psychiatry and Psychology: Science or Pseudoscience? |journal=[[Psychiatric Quarterly]] |language=en |volume=73 |issue=1 |pages=71–82 |doi=10.1023/a:1012896802713 |pmid=11780600 |s2cid=7229563 |issn=0033-2720}}</ref><ref>{{Cite journal |last=Charney |first=Evan |author-link=Evan Charney|date=2016-12-01 |title=Genes, behavior, and behavior genetics |journal=[[Wiley Interdisciplinary Reviews: Cognitive Science]] |language=en |volume=8 |issue=1–2 |pages=e1405 |doi=10.1002/wcs.1405 |pmid=27906529 |issn=1939-5078|hdl=10161/13337 |hdl-access=free }}</ref> On this basis, critics contend that twin studies tend to generate inflated or deflated estimates of heritability due to biological [[confounding factor]]s and consistent underestimation of environmental variance.<ref name=burtsimons/><ref>{{Cite journal |last=Segalowitz |first=Sidney J. |author-link=Sidney Segalowitz|date=October 1999 |title=Why twin studies really don't tell us much about human heritability |journal=[[Behavioral and Brain Sciences]] |language=en |volume=22 |issue=5 |pages=904–905 |doi=10.1017/S0140525X99442207 |s2cid=143865688 |issn=1469-1825}}</ref> Other critics take a more moderate stance, arguing that the equal environments assumption is typically inaccurate, but that this inaccuracy tends to have only a modest effect on heritability estimates.<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}}</ref>

=== Criticisms of statistical methods ===
[[Peter Schönemann]] criticized methods for estimating [[heritability]] developed in the 1970s. He has also argued that the heritability estimate from a twin study may reflect factors other than shared [[genes]]. Using the statistical models published in [[John C. Loehlin|Loehlin]] and Nichols (1976),<ref>{{cite book | last1 = Loehlin | first1 = John C. | author-link1 = John C. Loehlin | last2 = Nichols | first2 = Robert | title = Heredity, environment, & personality: a study of 850 sets of twins | publisher = [[University of Texas Press]] | location = [[Austin, Texas|Austin]] | year = 1976 | isbn = 978-0-292-73003-8 | jstor = 2826060 | url = https://archive.org/details/heredityenvironm0000loeh }}</ref> the narrow HR-heritability of responses to the question "did you have your back rubbed"  has been shown to work out to .92 heritable for males and .21 heritable for females, and the question "Did you wear sunglasses after dark?" is 130% heritable for males and 103% for females.<ref>{{Cite journal | last1 = Schönemann | first1 = Peter H. | author-link1 = Peter Schönemann| title = On models and muddles of heritability | journal = [[Genetica]]| volume = 99 | issue = 2–3| pages = 97–108| doi = 10.1023/A:1018358504373 | year = 1997 | pmid = 9463078| s2cid = 37292855 }}</ref><ref>{{cite conference | last1 = Schönemann | first1 = Peter H. | author-link1 = Peter Schönemann | year = 1995 | title = Totems of the IQ Myth: General Ability (g) and its Heritabilities (h<sup>2</sup>, HR) | conference = 1995 Meetings of the American Association for the Advancement of Sciences }}</ref> Critics also contend that the concept of "heritability" estimated in twin studies is merely a statistical abstraction with no relationship to the underlying DNA<ref>{{Cite journal |last1=Pam |first1=A. |last2=Kemker |first2=S. S. |last3=Ross |first3=C. A. |last4=Golden |first4=R. |date=July 1996 |title=The "Equal Environments Assumption" in MZ-DZ Twin Comparisons: an Untenable Premise of Psychiatric Genetics? |journal=Acta Geneticae Medicae et Gemellologiae: Twin Research |language=en |volume=45 |issue=3 |pages=349–360 |doi=10.1017/S0001566000000945 |pmid=9014000 |s2cid=34798197 |issn=0001-5660}}</ref> and that the [[statistical]] underpinnings of twin research are invalid. Statistical critiques argue that [[heritability]] estimates used for most twin studies rest on restrictive assumptions that are usually not tested, and if they are, they are often contradicted by the data.

==== Responses to statistical critiques ====

Before computers, statisticians used methods that were computationally tractable, at the cost of known limitations. Since the 1980s these approximate statistical methods have been discarded. Modern twin methods based on [[structural equation modeling]] are not subject to the limitations and heritability estimates such as those noted above are mathematically impossible.<ref>M. C. Neale, S. M. Boker, G. Xie and H. H. Maes. (2002). Mx: Statistical Modelling. ''Journal''.</ref>
Critically, the newer methods allow for explicit testing of the role of different pathways and incorporation and testing of complex effects.<ref name=Neale1996 />

=== Sampling: Twins as representative members of the population ===
Results of twin studies cannot be automatically generalized beyond the population they come from. It is therefore important to understand the particular sample studied, and the nature of twins themselves. Twins are not a [[random sample]] of the population, and they differ in their developmental environment. In this sense they are not representative.<ref>{{Cite journal | last1 = Record | first1 = R. G.| last2 = McKeown | first2 = Thomas| last3 = Edwards | first3 = J. H.| doi = 10.1111/j.1469-1809.1970.tb00215.x | title = An investigation of the difference in measured intelligence between twins and single births | journal = [[Annals of Human Genetics]]| volume = 34 | issue = 1 | pages = 11–20 | year = 1970 | pmid =  5529232| s2cid = 30109683}}</ref>

For example: Dizygotic (DZ) twin births are affected by many factors. Some women frequently produce more than one [[ovum|egg]] at each [[menstrual]] period and are therefore more likely to have twins. This tendency may run in the [[family]] either on the mother's or father's side, and often runs through both. Women over the age of 35 are more likely to produce two eggs. Women who have three or more children are also likely to have dizygotic twins. Artificial induction of [[ovulation]] and in [[vitro]] [[fertilization]]-[[embryo]] replacement can also give rise to fraternal and identical twins.<ref>{{cite book | last1 = Woollett | first1 = Anne | last2 = Clegg | first2 = Averil | title = Twins: from conception to five years | publisher = Century | location = London | year = 1983 | page = 127 | isbn = 978-0-7126-0204-4 | oclc = 12445470 }}</ref><ref>{{Cite journal
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}}</ref><ref>{{Cite journal | last1 = Derom | first1 = C.| last2 = Derom | first2 = R. | last3 = Vlietinck | first3 = R. | last4 = Berghe | first4 = H. V. | last5 = Thiery | first5 = M. | title = Increased Monozygotic Twinning Rate After Ovulation Induction | doi = 10.1016/S0140-6736(87)92688-2 | journal = [[The Lancet]]| volume = 329 | issue = 8544 | pages = 1236–1238| date=May 1987 | pmid = 2884372| s2cid = 919014}}</ref><ref>{{Cite journal | last1 = Edwards | first1 = R. G. | last2 = Mettler | first2 = L. | last3 = Walters | first3 = D. E. | doi = 10.1007/BF01139357 | title = Identical twins and in vitro fertilization | journal = [[Journal of Assisted Reproduction and Genetics|Journal of In Vitro Fertilization and Embryo Transfer]] <!-- current name and prior to 1992 -->| volume = 3 | issue = 2 | pages = 114–117 | date=April 1986 | pmid =  3701181| s2cid = 27421288 }}</ref><ref>{{cite book | last = Leigh | first = Gillian | year = 1983 | title = All About Twins: A Handbook for Parents | location = London | publisher = [[Routledge and Kegan Paul]] | isbn = 978-0-7100-9888-7 | url = https://archive.org/details/allabouttwins00gill_0 }}</ref><ref>{{cite journal | first1 = Christiane | last1 = Capron | first2 = Adrian R. | last2 = Vetta | first3 = Michel | last3 = Duyme | first4 = Atam | last4 = Vetta | year = 1999 | title = Misconceptions of biometrical IQists | journal = Cahiers de Psychologie Cognitive/Current Psychology of Cognition | volume = 18 | issue = 2 | pages = 115–160 }}</ref>

==== Response to representativeness of twins ====

However, twins differ very little from non-twin siblings. Measured studies on the personality and intelligence of twins suggest that they have scores on these traits very similar to those of non-twins (for instance Deary et al. 2006).

=== Separated twin pairs as representative of other twins ===
Separated twin pairs, identical or fraternal, are generally separated by [[adoption]]. This makes their families of origin non-representative of typical twin families in that they give up their children for adoption. The families they are adopted to are also non-representative of typical twin families in that they are all approved for adoption by children's protection authorities and that a disproportionally large fraction of them have no biological children. Those who volunteer to studies are not even representative of separated twins in general since not all separated twins agree to be part of twin studies.<ref>Fatal Flaws in the Twin Study Paradigm: A Reply to Hatemi and Verhulst, Doron Shultziner 2013</ref><ref>Twin Studies of Political Behavior: Untenable Assumptions?, Jon Beckwith and Corey A. Morris 2008</ref>

=== Detection problems ===
There can be some issues of undetected behaviors in the case of behaviors that many people keep secret presently or in their earlier lives. They may not be as willing to reveal behaviors that are discriminated against or stigmatized. If environment played no role in the actual behavior, skewed detection would still make it look like it played a role. For environment to appear to have no role in such cases, there would have to be either a counterproductivity of intolerance in the sense of intolerance causing the behavior it is bigoted against, or a flaw in the study that makes the results scientifically useless. Even if environment plays no role, the numbers would still be skewed.<ref>Critical Analysis: A Comparison of Critical Thinking Changes in Psychology and Philosophy Classes, Teaching of Psychology 2014 41: 28</ref><ref>Association for Psychological Science: Why Science Is Not Necessarily Self-Correcting, John P. A. Ioannidis 2012</ref><ref>How Black African and White British Women Perceive Depression and Help-Seeking: a Pilot Vignette Study, International Journal of Social Psychiatry March 2010</ref>