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Genes, Twin Studies

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Extent of genetic effects

  • 60% (50-70) general intelligence
  • 50% extraversion
  • 40% occupational interest
  • 45% whether ever married
  • 35% (30-40) social attitudes
  • 35-50% homosexuality in men (35% SGU 5x5 #26)
    • Likelihood if identical twin gay = 50%, likelihood if fraternal twin gay = 25% (Nat'l Geographic)
    • Testosterone release at about 8 weeks, epigenetic switches (enzymes that attach to DNA and activate or deactivate genes) (ibid)
  • 30% job satisfaction
  • 21% whether divorced or never divorced
  • 18% homosexuality in women (SGU 5x5 #26)
  • 0% love styles (whether we fall in love gradually or are "swept off our feet")


IQ Correlation

  • .86 - Identical twins reared together (approximately the same as one person taking test twice)
  • .75 - Identical twins reared apart
  • .60 - fraternal twins
  • .42 - parents and children
  • .29-.34 - adoptive young siblings, moves to zero as adults
  • .15 - cousins
    • (EL p50)


Extent that genetic differnces explain IQ differences

  • 20% infancy
  • 40% chilhood
  • 50% adolescence
  • 60% adulthood (non-shared environments account for 30%)


  • "A surprise was that traditionalism, the endorsement of traditional family and moral values, did not show common family effects. In other words, living with someone does not lead to agreement on standards of conduct or parenting practices." (EL p76)
  • "The bottom line is that living together does not make people in a family alike and that similarities are explained primarily by shared genes." (EL p76)
  • Average IQ of twins = 90-95; 80% of identifical twins within 10 points in IQ, remaining differences largely due to trauma (EL p58)


Misc Twins notes

  • Monozygotic twins = about 1/250 pregnancies (fairly consistent across cultures, apparently random)
  • Dizygotic twins = about 1/125 pregancies (and some have different fathers)
  • "We also know that reared-apart identical twins are as alike in personality as reared-together identifical twins."
  • Remember that fraternal twins (and all siblings) share an average of 50% of their genes (and that most cells they inherit from their mother are not genetically transfered)




US Births 1996

  • 3,914,953 live births
  • 100,750 born as twins
  • 5,298 born as triplets (1,766 sets)
  • 560 born as quadruplets (140 sets)
  • 81 born as quintuplets or greater



Fraternal twins born more often to

  • Older women, more common among mothers 35-39
  • Taller and heavier women
  • Last born children
  • Conceived after periods of abstinence
  • Conceived after higher frequency of sex



  • Genetic left-handers function normally but are more likely to process language (at least in part) in the right hemisphere. Because many of these left-handers are part of a monozygotic twin pair, the situs inversus phenomenon (mirror-image twinning) is probably involved. It has been suggested that when a left-hander is born as a singleton the conception was originally a monozygotic twin set, but one twin, the right-handed one, died and was reabsorbed in utero. This is plausible, but highly speculative. It has also been suggested that there is a genetic component to monozygotic twinning and, if this is true, it could provide a genetic basis for a small proportion of left-handers.


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