Editing Twin study (section)

=== 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
| last1 = Corson | first1 = S. L.
| last2 = Dickey | first2 = R. P.
| last3 = Gocial | first3 = B.
| last4 = Batzer | first4 = F. R.
| last5 = Eisenberg | first5 = E.
| last6 = Huppert | first6 = L.
| last7 = Maislin | first7 = G.
| title = Outcome in 242 in vitro fertilization-embryo replacement or gamete intrafallopian transfer-induced pregnancies
| journal = [[Fertility and Sterility]]
| volume = 51
| issue = 4
| pages = 644–650
| year = 1989
| pmid = 2924931
| doi = 10.1016/S0015-0282(16)60614-3
| doi-access = free
}}</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).