Editing Twin study (section)

==== Multi-group and multivariate modeling ====
Multivariate modeling can give answers to questions about the genetic relationship between variables that appear independent. For instance: do IQ and long-term memory share genes? Do they share environmental causes? Additional benefits include the ability to deal with interval, threshold, and continuous data, retaining full information from data with missing values, integrating the latent modeling with measured variables, be they measured environments, or, now, measured molecular genetic markers such as [[Single-nucleotide polymorphism|SNPs]]. In addition, models avoid constraint problems in the crude correlation method: all parameters will lie, as they should, between 0–1 (standardized).

Multivariate, and multiple-time wave studies, with measured environment and repeated measures of potentially causal behaviours are now the norm. Examples of these models include extended twin designs,<ref>{{Cite journal | last1 = Keller | first1 = M. C.| last2 = Medland | first2 = S. E. |author-link2=Sarah Medland |last3 = Duncan | first3 = L. E.| doi = 10.1007/s10519-009-9320-x | title = Are Extended Twin Family Designs Worth the Trouble? A Comparison of the Bias, Precision, and Accuracy of Parameters Estimated in Four Twin Family Models | journal = [[Behavior Genetics (journal)|Behavior Genetics]] | volume = 40 | issue = 3 | pages = 377–393 | date=May 2010 | pmid =  20013306| pmc = 3228846 }}</ref><ref>{{Cite journal | last1 = Coventry | first1 = W. L. | last2 = Keller | first2 = M. C. | doi = 10.1375/1832427054253121 | title = Estimating the Extent of Parameter Bias in the Classical Twin Design: A Comparison of Parameter Estimates from Extended Twin-Family and Classical Twin Designs | journal = [[Twin Research and Human Genetics]] | volume = 8 | issue = 3 | pages = 214–223 | date = June 2005 | pmid = 15989749 | url = http://e-publications.une.edu.au/1959.11/6460 | doi-access = free }}{{Dead link|date=October 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> simplex models,<ref>{{Cite journal | last1 = Gillespie | first1 = Nathan A. | last2 = Evans | first2 = David E. | last3 = Wright | first3 = Margie. M. | last4 = Martin | first4 = Nicholas G. | title = Genetic Simplex Modeling of Eysenck's Dimensions of Personality in a Sample of Young Australian Twins | doi = 10.1375/1369052042663814 | journal = [[Twin Research and Human Genetics|Twin Research]] | volume = 7 | issue = 6 | pages = 637–648 | year = 2004 | pmid = 15607015 | url = http://www.vipbg.vcu.edu/~nathan/publications/Gillespie2005b.pdf | access-date = 2010-11-05 | archive-url = https://web.archive.org/web/20100624040406/http://www.vipbg.vcu.edu/~nathan/publications/Gillespie2005b.pdf | archive-date = 2010-06-24 | url-status = dead }}</ref> and growth-curve models.<ref>{{Cite journal | last1 = Neale | first1 = M. C.| last2 = McArdle | first2 = J. J.| doi = 10.1375/136905200320565454 | title = Structured latent growth curves for twin data | journal = [[Twin Research and Human Genetics|Twin Research]]| volume = 3 | issue = 3 | pages = 165–177| date=September 2000 | pmid = 11035490| url = http://www.vipbg.vcu.edu/vipbg/Articles/TwinRes-structured-2000.pdf| citeseerx = 10.1.1.336.1002}}</ref>

[[Structural equation modeling|SEM]] programs such as [[OpenMx]]<ref>{{Cite journal | last1 = Boker | first1 = Steven| last2 = Neale | first2 = Michael| last3 = Maes | first3 = Hermine| last4 = Wilde | first4 = Michael| last5 = Spiegel | first5 = Michael| last6 = Brick | first6 = Timothy| last7 = Spies | first7 = Jeffrey| last8 = Estabrook | first8 = Ryne| last9 = Kenny | first9 = Sarah| last10 = Bates | first10 = Timothy| last11 = Mehta | first11 = Paras| last12 = Fox | first12 = John| doi = 10.1007/s11336-010-9200-6 | title = OpenMx: An Open Source Extended Structural Equation Modeling Framework | journal = [[Psychometrika]] | volume = 76 | issue = 2 | pages = 306–317| year = 2011 | pmid =  23258944| pmc = 3525063}}</ref> and other applications suited to constraints and multiple groups have made the new techniques accessible to reasonably skilled users.