In preparing our new site I got sidetracked looking at twin data. As a gay man and a father of fraternal twins, it interests me (and some of the numbers are just funny). The following data, if a single percentile, are hereditability figures. Thus 100% would mean the trait is completely genetically hereditable. Some other data below comes in concordance form and so the identical twin concordance (siblings with the same genes ) is first and the fraternal twin (siblings with different genes) is second. Thus, if the identical twin’s concordance is 100% and the fraternal is 0% one could near safely say genetics is the cause of the trait.
Eye color: 87% (1)
It surprised me this isn’t nearer 100%. I mean, I was under the impression, if it's not genetic, then it's a choice... ;-) Why are people choosing their eye color to be different from their twin?
Crypts in eye: 75% (1)
Those are specks in your brother’s eye…
Height: 88% (2)
Weight: 88% (2)
Left Handedness: 15% vs. 9% (3) (concordance)
See this post for more on handedness , or about my disdain for those who practice such a lifestyle here.
Onset of Puberty: 50-80% (4)
The important trigger to most of our realizations and experiences of our sexual orientations, gay or straight.
Depression: 39% (5)
Chance of Stroke: 32% (6)
Stroke Hospitalization: 17% (6)
Prostate Cancer: 19% vs 4% (7) (concordance)
I’m surprised that last one is so low.
Opinion and Personality:
All the following are from reference 8, and all are concordance numbers for identical vs fraternal twins. Also, I’ve just picked the interesting ones out and there are more if if you’d like to look at the paper.
Death Penalty Opinion: 45% vs 33%
Open-door immigration: 47% vs 20%
Separate Roles For Men and Women: 27% vs. 26%
Odd, seems little genetic involvement in gender role opinion...
Voluntary euthanasia: 45% vs 21%
Making racial discrimination illegal 37% vs. 1% (Big difference here)
Capitalism 41% vs. 19%
Abortion on demand 53% vs. 28%
Easy access to birth control 24% vs. 27% (Now why would that be?)
Organized religion 43% vs. 21%
Castration as punishment for sex crimes 39% vs. 29%
Doing Crossword puzzles: 46% vs 11% (Hey, that’s near the numbers for homosexuality :-))
Being the leader of groups 40% vs. 8%
Being assertive 28% vs. 27%
Humble 60% vs. 27%
Ambitious 46% vs. 24%
Aggressive 27% vs. 09%
Dominant 24% vs. 12%
Exhibitionistic 44% vs. 23%
Fearful 26 % vs. 01% (seems there’s a big genetic effect in anxiety?)
Obliging 11 % vs. 18%
Inquiring 22% vs. 13%
Athletic 60% vs. 27%
Physically Strong 52% vs. 21%
GPA: 61% vs 25%
Level of Education: 82% vs 57%
Political and Social attitude in all: 36% (8) hereditability when all opinions are averaged.
Political and Social attitude in all from a different, repeat study: 18-41% (9) (depending on the particular issue from abortion to immigration)
Being a Republican: 36% :-) (9)
A Socialist: 36%
Isn’t that a pretty big hereditability for something we take as stridently as politics :-)?
Gay Rights: 28% genetic hereditability in opinion on gay rights. LOL, Some just can’t help being queer lovers or homophobes; they’re born that way :-).
1. Larsson, M., N. L. Pedersen, et al. (2003). "Importance of genetic effects for characteristics of the human iris." Twin Research 6(3): 192-200.
2. Carmichael, C. M. and M. McGue (1995). "A cross-sectional examination of height, weight, and body mass index in adult twins." Journal of Gerontology Series A: Biological and Medical Sciences 50(4): B237-B244.
3. Sicotte, N., R. Woods, et al. (1999). "Handedness in Twins: A Meta-analysis." Laterality: Asymmetries of Body, Brain, and Cognition 4(3): 265-286.
4. Palmert, M. R. and P. A. Boepple (2001). "Variation in the Timing of Puberty: Clinical Spectrum and Genetic Investigation." The Journal of Clinical Endocrinology and Metabolism 86(6): 2365-2368.
5. Kendler, K. S. and C. A. Prescott (1999). "A Population-Based Twin Study of Lifetime Major Depression in Men and Women." Archives of General Psychiatry 56: 39-44.
6. Bak, S., D. Gaist, et al. (2002). "Genetic liability in stroke: a long-term follow-up study of Danish twins." Stroke 33(3): 769-774.
7. Gronberg, H., L. Damber, et al. (1994). "Studies of genetic factors in prostate cancer in a twin population." Journal or Urology 152(5): 1484-1489.
8. Olson, J. M., P. A. Vernon, et al. (2001). "The Heritability of Attitudes: A Study of Twins." Journal of Personality and Social Psychology 80(6): 845-860.
9. Alford, J. R., C. L. Funk, et al. (2005). "Are Political Orientations Genetically Transmitted?" American Political Science Review 99(2): 153-167.