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Am J Psychiatry 157:11, November 2000 1843
Article
Sexual Orientation in a U.S. National Sample
of Twin and Nontwin Sibling Pairs
Kenneth S. Kendler, M.D.
Laura M. Thornton, Ph.D.
Stephen E. Gilman, S.M.
Ronald C. Kessler, Ph.D.
Objective: Although previous studies have
suggested that sexual orientation is influ-
enced by familial factors, which may be
partly genetic, these studies have relied
on unrepresentative and potentially bi-
ased samples. The authors attempted to
estimate the role of genetic and environ-
mental factors in the determination of
sexual orientation in a more representa-
tive sample.
Method: Sexual orientation was assessed
by a single item on a self-report question-
naire in a U.S. national sample of twin and
nontwin sibling pairs. Sexual orientation
was classified as heterosexual or nonhet-
erosexual (bisexual or homosexual). The
authors compared the similarity of sexual
orientation in the monozygotic twins to
the similarity in the same-sex dizygotic
twins, all dizygotic twins, the same-sex di-
zygotic twins and sibling pairs, and all dizy-
gotic twins and sibling pairs. Biometrical
twin analyses were performed.
Results: All analyses demonstrated fa-
milial resemblance for sexual orientation.
Resemblance was greater in the mono-
zygotic twins than in the dizygotic twins
or in the dizygotic twins plus nontwin
siblings. Biometrical twin modeling sug-
gested that sexual orientation was sub-
stantially influenced by genetic factors,
but family environment may also play a
role. No evidence was found for a viola-
tion of the equal-environment assump-
tion regarding monozygotic and dizygotic
twin pairs.
Conclusions: Familial factors, which are
at least partly genetic, influence sexual
orientation. The results of these analyses
should be interpreted in the context of
low statistical power and the use of a sin-
gle item to assess the complex phenotype
of sexual orientation.
(Am J Psychiatry 2000; 157:1843–1846)
The etiology of sexual orientation has been a focus of
both scientific and general social interest. Twin studies (1–
4) have suggested that familial factors, which may be ge-
netic, have a substantial impact on sexual orientation.
Three studies (5–7), with conflicting results, have exam-
ined whether a locus on the X chromosome influences
sexual orientation.
The interpretation of prior twin studies of sexual orien-
tation can be questioned on methodological grounds.
Kallmann’s early study (1) largely sampled subjects from
correctional and psychiatric institutions. The two studies
of Bailey and colleagues on male (2) and female (3) sexual
orientation obtained subjects through advertisements in
homophilic publications, raising concerns about repre-
sentativeness. To our knowledge, only one study of sexual
orientation has used a general population volunteer twin
registry (4). We here report results on sexual orientation in
twin and nontwin sibling pairs ascertained as part of a
U.S. national probability sample.
Method
Sampling
The data come from the MacArthur Foundation Midlife Devel-
opment in the United States survey (8, 9). This is a national tele-
phone/mail survey carried out in 1995–1996 under the auspices
of the John D. and Catherine T. MacArthur Foundation Network
on Successful Midlife Development. A sample of 3,032 respon-
dents, ranging in age from 25 to 74 years, was recruited from a
random-digit-dial sampling frame of the coterminous United
States. Only one respondent was selected from each eligible
household. The survey was carried out in two phases: a telephone
interview fo llowed by a self-ad ministered mail questionnaire. The
phase 1 response rate was 70.0%, and the conditional phase 2 re-
sponse rate was 86.8%, with an overall response rate of 60.8%. The
entire protocol, including the obtaining of informed consent
through oral assent before initiation of the telephone interview,
was reviewed and approved by the Human Subjects Committee of
Harvard University Medical School.
Twin pairs were recruited by using a separate two-part sam-
pling design, the first part of which i nvolved screening a represen-
tative national sample of approximately 50,000 households for
the presence of a twin. This was done as part of ongoing national
omnibus surveys conducted by ICR/AUS Consultants and
Bruskin Associates. Respondents who indicated the presence of
twins in the household or being part of a twin pair themselves
were asked permission to be contacted by our research team for
inclusion in the first national study of twins. The presence of a
twin in the family was reported by 14.8% of the respondents, of
whom 60.0% gave permission to be contacted for the twin study.
The second part of the twin sample design involved student re-
cruiters from the University of Michigan contacting the twin
households in order to recruit twins to participate in the survey.
The cooperating twins were asked to provide contact information
for their co-twins, who were also recruited by the students. The fi-
1844 Am J Psychiatry 157:11, November 2000
SEXUAL ORIENTATION
nal response rate for the twin pairs varied according to whether
the first contact was with a relative of the twin (20.6% response
rate) or the twin him- or herself (60.4% response rate). The final
twin sample included a total of 1,588 twins, resulting in 794 pairs,
763 of which came from distinct families. Fourteen families con-
tributed two twin pairs, while one family contributed three twin
pairs. Of these 794 pairs, 756 had a known zygosity and informa-
tion on sexual orientation from both members.
Nontwin siblings were enrolled by sending a postcard to all re-
spondents to the MacArthur Foundation Midlife Development in
the United States survey, telling them of our interest in including
siblings in the survey. The card asked them to provide contact in-
formation for their siblings and to communicate with their sib-
lings about participation before the time when a recruiter would
attempt contact. Since the family study was a secondary aim of the
project, aggressive follow-up procedures were not employed.
While the number of eligible respondents who provided us with
the names and addresses of their siblings was low (19.7%), the co-
operation rate for the sibling sample was much higher (69.3%). Of
the 1,372 siblings referred to us from the survey participants, 951
were interviewed. The number of referred siblings recruited from
any single family ranged from one to six, including one sibling
from 272 families, two from 146 families, three from 75 families,
four from 22 families, five from 10 families, and six from four fam-
ilies. Thus, these 951 siblings came from 529 families of original
participants in the MacArthur Foundation Midlife Development
in the United States survey, for a total sample of 1,480. However, of
these newly recruited siblings, 81 did not complete the self-report
questionnaire. Removing them from the sample left 19 individuals
with no sibling, who were also excluded from the data analysis, so
the final sample of nontwin siblings included 1,380 individuals.
Subject Characteristics
The twin and nontwin sibling subjects ranged in age from 25 to
74 years, with a mean of 47.2 (SD=12.6). The majority of the sub-
jects were white (95.1%), and they were diverse with respect to ed-
ucational attainment: 33.5% had graduated from college, 29.6%
had some college education, 27.7% had graduated from high
school, and the remaining 9.3% had less than a high school edu-
cation. Among the nontwin sibling pairs, the mean age difference
was 5.8 years (SD=4.5).
Assessment of Sexual Orientation
Sexual orientation was assessed by a single item in part 2 of the
self-report questionnaire that read, “How would you describe
your sexual orientation? Would you say you are heterosexual (sex-
ually attracted only to the opposite sex), homosexual (sexually at-
tracted only to your own sex), or bisexual (sexually attracted to
both men and women)?” The response options were 1) heterosex-
ual, 2) homosexual, and 3) bisexual. Because of limited statistical
power, for the purpose of our present analyses we combined re-
sponses 2 and 3 to create a dichotomous rating of sexual orienta-
tion: heterosexual and nonheterosexual.
Zygosity Determination
Using as a test sample 230 pairs of unselected genotyped adult
same-sex twins from the Virginia Twin Registry, we examined the
common set of eight standard zygosity self-report items that were
identical or highly similar across the two data sets. We obtained a
linear discriminant function from the “test” sample of our 230
genotyped pairs and then applied it to the same-sex twin pairs in
the sample from the MacArthur Foundation Midlife Develop-
ment in the United States survey. Good separation was obtained,
as 86% of the pairs were assigned a probability of monozygosity of
less tha n 10% or mo re than 9 0%. We lef t as una ssigned the 3. 5% of
pairs with a probability of monozygosity of between 40% and
59%, and these pairs were excluded from further data analysis.
Statistical Analysis
We assessed twin (and sibling) resemblance in three ways.
Proband-wise concordance was defined as the proportion of co-
twins of nonheterosexual twins who were themselves nonhetero-
sexual. We also calculated the odds ratio, which is the ratio of the
odds of being nonheterosexual among co-twins of nonheterosex-
ual twins to the odds of being nonheterosexual among co-twins
of heterosexual twins.
We used a liability-threshold model to estimate the genetic and
environmental contributions to sexual orientation. For categori-
cal characteristics, such as sexual orientation, the estimates are
for the resemblance of twins in a pair for their liability to develop
a nonheterosexual versus heterosexual sexual orientation (10). Li-
ability is assumed to be continuous and normally distributed in
the population, with individuals who exceed a theoretical thresh-
old expressing a nonheterosexual sexual orientation. We also
present the tetrachoric correlation, defined as the correlation in
members of twin or sibling pairs for the liability to nonheterosex-
ual versus heterosexual sexual orientation (10, 11).
We initially assumed four sources of individual differences in
liability in our modeling: additive genes (abbreviated “A”), com-
mon or familial environment that affects the similarity of all twin/
sibling pairs (C), a special twin environment that influences the
similarity of twins only (T), and individual-specific environment
(E). However, in all our models, estimates of T were zero because,
contrary to expectation, sibling resemblance for sexual orienta-
tion was modestly greater than that seen in the dizygotic twins.
Therefore, where necessary, we combined dizygotic twins and
siblings into a single category and estimated only A, C, and E.
Models were fit with maximum likelihood estimation by using the
Mx structural modeling program (12). Because of our low power
for discriminating between models, we chose to present parame-
ter estimates from only the full (ACE) model and divided, in our
analyses, multiple sibships into all possible sibling pairs. This ap-
proach can be expected to obtain accurate parameter estimates
but may underestimate sampling errors (13).
The equal-environment assumption requires that members of
monozygotic and dizygotic twin pairs be equally similar in their
exposure to environmental factors of etiologic relevance to the
trait being studied. If the environments of dizygotic twins were
less similar than those of monozygotic twins, this would lead to an
overestimation of genetic influence. We examined this possibility
in two ways: by examining standard measures of the environmen-
tal similarity of twins in childhood (14) and by examining the fre-
quency of current contact between the members of the twin pair
in adulthood. Using logistic regression and controlling for zygos-
ity, we examined whether the mean level of childhood or adult en-
vironmental similarity reported by the twin pair could predict the
concordance status of the twin pair for sexual orientation.
Because of the rarity of nonheterosexual sexual orientation in
this sample, we had no statistical power to examine gender differ-
ences in sexual orientation. Whether similar or distinct familial fac-
tors influence sexual orientation in males and females remains un-
certain (4, 15). Therefore, we conducted analyses with and without
opposite-sex dizygotic twins and nontwin siblings. Furthermore, we
performed analyses by using the traditional twin design (monozy-
gotic and dizygotic twins) as well as maximizing our power by in-
cluding together dizygotic twins and nontwin siblings (who both
share, on average, 50% of their genes identical by descent).
Results
Among the total sample of 2,968 individuals, data on
sexual orientation were missing for 61 (2.1%). Of the re-
maining 2,907, 81 (2.8%) reported a nonheterosexual sex-
ual orientation. The rate was nonsignificantly higher in
Am J Psychiatry 157:11, November 2000 1845
KENDLER, THORNTON, GILMAN, ET AL.
men (3.1%) than in women (2.5%) (χ2=1.10, df=1, p=0.30).
With controls for age and gender, the rate of nonhetero-
sexual sexual orientation did not significantly differ be-
tween the twins and nontwin siblings (χ2=0.18, df=1, p=
0.92), between the monozygotic and dizygotic twins (χ2=
0.04, df=1, p=0.93), or between the same-sex and oppo-
site-sex dizygotic twins (χ2=0.44, df=1, p=0.51). When we
controlled for zygosity, twin resemblance for sexual orien-
tation was not significantly predicted by measures of the
similarity of their childhoods (χ2=1.02, df=1, p=0.31) or
adult environments (χ2=2.47, df=1, p=0.12).
As assessed by proband-wise concordance, odds ratios,
or tetrachoric correlations, pair resemblance for sexual
orientation was substantially greater in the monozygotic
twins than in any of the groups of dizygotic twins and non-
twin siblings (Table 1). Both in the dizygotic twin pairs
alone and in the dizygotic twins plus nontwin sibling
pairs, the resemblance for sexual orientation was greater
in the same-sex pairs than in the opposite-sex pairs.
We performed biometrical twin modeling with the
monozygotic twins and each of the four possible compar-
ison groups of dizygotic twins and nontwin sibling pairs.
Using the full model, we found estimates of the heritability
of liability of sexual orientation ranging from 0.28 to 0.65.
The estimates of the impact of familial environment were
lower and ranged from 0.00 to 0.39.
Discussion
As in previous studies (e.g., references 15, 16), sexual
orientation, as assessed by self-report questionnaire in a
U.S. national sample of twin and sibling pairs, demon-
strated substantial familial aggregation. In accord with
findings from prior twin studies (1–4), resemblance for
sexual orientation was greater in monozygotic twins than
in dizygotic twins or nontwin sibling pairs. These results
suggest that genetic factors may provide an important in-
fluence on sexual orientation. The concordance rate for
nonheterosexual sexual orientation in monozygotic twins
found in this sample (31.6%) is similar to that found in the
one previous general population twin study (4) but lower
than the approximately 50% concordance rates found in
the two previous studies that ascertained twin pairs
through homophilic publications (2, 3). These results sug-
gest that twin pairs concordant for sexual orientation may
be more likely to respond to such ads than are twin pairs
discordant for sexual orientation.
These results should be interpreted in the context of im-
portant methodologic strengths and weaknesses of this
sample. Three strengths are noteworthy. First, in contrast
to prior twin studies of sexual orientation, this study was
based on a national probability sample. One previous
study used a general population sample (4), but that was
derived from a volunteer twin registry. Second, we as-
sessed sexual orientation by questionnaire, which, with its
relative anonymity, may be more likely than personal in-
terview to elicit accurate responses on something as sensi-
tive as sexual orientation (17–19). In their large survey of
sexual practices in the United States, Laumann et al. (20)
included an item similar to the one used in our survey as
part of their self-report questionnaire. They reported rates
of nonheterosexual sexual orientation that were identical
to that found in this sample for men (3.1%) but somewhat
lower for women (1.5%). It seems unlikely that we have
underestimated the population prevalence of nonhetero-
sexual sexual orientation. Third, we were able to explicitly
test for the equal-environment assumption, albeit with
low statistical power. We found no evidence that more
similar environmental experiences in monozygotic than
in dizygotic twins contributed to the greater resemblance
for sexual orientation in the monozygotic pairs.
However, this sample also has five potentially important
limitations. First, the assessment of the complex pheno-
type of sexual orientation with a single item is far from
TABLE 1. Nonheterosexual Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs
Group
Number
of
Pairs
Prevalence
of Non-
heterosexual
Sexual
Orientation
(%)
Probandwise Concordance
for Nonheterosexual Sexual
Orientation
Odds
Ratioa
Tetrac horic
Correlation
(r)b
Origins of Liability to Nonheterosexual
Sexual Orientationc
Pairs With
Any Non-
heterosexual
Member
Concordant
Pairs
Additive
Genetic
Effects
(a2)
Common or
Familial
Environmental
Effects (c2)
Individual-
Specific
Environmental
Effects (e2)Number %
Monozygotic twins 324 3.0 19 6 31.6 23.1 +0.68
Same-sex dizygotic
twins 240 3.1 15 2 13.3 5.7 +0.37 0.62 0.05 0.33
All dizygotic twins 432 2.8 24 2 8.3 3.4 +0.25 0.65 0.00 0.35
Same-sex dizygotic
twins and non-
twin sibling pairs 950 2.9 56 12 21.4 11.4 +0.53 0.28 0.39 0.33
All dizygotic twins
and nontwin
sibling pairs 1,806 2.6 93 14 15.1 7.9 +0.43 0.45 0.21 0.34
aRatio of the odds of being nonheterosexual among co-twins of nonheterosexual twins to the odds of being nonheterosexual among co-twins
of heterosexual twins.
bCorrelation between twins or siblings for the liability to a nonheterosexual sexual orientation.
cEstimates are from the full model.
1846 Am J Psychiatry 157:11, November 2000
SEXUAL ORIENTATION
ideal. Sexual orientation involves, at a minimum, dimen-
sions of sexual fantasy, attraction, and behavior, none of
which was explicitly captured by our single item. Second,
despite the reasonable sample sizes, the relative rarity of
nonheterosexual sexual orientation in general population
samples results in quite low statistical power. For example,
the 95% confidence intervals (CIs) around the estimates of
a2 in Table 1 all include zero. If we examine the AE sub-
model (which provides the best fit for three of the four
analyses), we gain in the accuracy of parameter estima-
tion. However, even here, the 95% CIs for heritability range
widely—for example, from 31% to 89% for monozygotic
plus same-sex dizygotic twins. Third, our analytic method
did not account for the correlated observations in families
with more than two siblings. We reran the analysis of all
dizygotic pairs plus nontwin sibling pairs in a newly avail-
able option of Mx that corrected for the correlational
structure and obtained estimates from the full model (a2=
0.47, c2=0.20, e2=0.34) that were reassuringly similar to
those obtained in our initial analyses. Fourth, the small
number of cases of nonheterosexual sexual orientation
made it impossible to examine gender differences in the
causes of sexual orientation with any statistical power.
Qualitative comparisons indicated that the concordance
rate for nonheterosexual sexual orientation was higher in
the female-female pairs than in the male-male pairs
among the monozygotic twins (four of nine, or 44.4%, ver-
sus two of 10, or 20.0%) and for the dizygotic twins plus
nontwin sibling pairs (eight of 28, or 28.6%, versus four of
28, or 14.3%). The evidence that pair resemblance for sex-
ual orientation is greater in same-sex than in opposite-sex
pairs suggests that the familial factors that influence sex-
ual orientation may not be the same in males and females.
Fifth, our overall cooperation rates were only moderate for
the twin sample and even lower for the nontwin sibling
pairs. However, our evidence for heritable influences on
sexual orientation was somewhat stronger for analyses
that included only twins than for those that also included
siblings. This would not be the expected pattern if the evi-
dence for genetic effects on sexual orientation was due to
cooperation bias, indirectly arguing that the key finding is
substantive rather than due to a methodological artifact.
Received Dec. 22, 1999; accepted May 22, 2000. From the Depart-
ment of Psychiatry and the Department of H uman Genetics, Medical
College of Virginia/Virginia Commonwealth University; the Virginia
Institute for Psychiatric and Behavioral Genetics, Richm ond; the De-
partment of Health and Social Behavior, Harvard School of Public
Health, Boston; and the Department of Health Care Policy, Harvard
Medical School, Boston. Address reprint requests to Dr. Kendler, De-
partment of Psychiatry, Medical College of Virginia, P.O. Box 980126,
Richmond, VA 23298-0126.
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