Archives of Sexual Behavior pp700-aseb-456629 December 13, 2002 17:1 Style ﬁle version July 26, 1999
Archives of Sexual Behavior, Vol. 32, No. 1, February 2003, pp. 23–28 (
Finger-Length Ratios in Female Monozygotic Twins
Discordant for Sexual Orientation
Lynn S. Hall, Ph.D.,
and Craig T. Love, Ph.D.
Received October 11, 2001; revisions received May 31, 2002, and July 2, 2002; accepted July 2, 2002
The second to fourth ﬁnger digit ratio (2D:4D ratio) is a sex-dimorphic characteristic in humans that
may reﬂect relative levels of ﬁrst trimester prenatal sex hormones. Low interdigital ratio has been
associated with high levels of androgens. It has been reported in unrelated women that low 2D:4D
ratio is associated with lesbian sexual orientation, but because of the nature of those samples, it was
not possible to conclude whether lower ratio (and hypothetically, higher androgen levels) in lesbians
are due to differences in genetics as opposed to differences in environment. To test the hypothesis
that low 2D:4D in lesbians is due to differences in environment, interdigital ratio data were analyzed
in a sample of female monozygotic (MZ) twins discordant for sexual orientation (1 twin was lesbian,
the other was heterosexual; n = 7 pairs). A control group of female MZ twins concordant for sexual
orientation (both twins were lesbian) was used as a comparison (n = 5 pairs). In the twins discordant
for sexual orientation, the lesbian twins had signiﬁcantly lower 2D:4D ratios on both the right and
left hands than their heterosexual cotwins. There were no signiﬁcant differences for either hand in the
twins concordantforsexualorientation. BecauseMZ twins sharevirtually the samegenes, differences
in 2D:4D ratio suggest that low 2D:4D ratio is a result of differences in prenatal environment.
KEY WORDS: lesbian; monozygotic twins; interdigital ratio; sexual orientation.
The 2D:4D (second to fourth ﬁnger digit ratio) ratio
is a sex-dimorphic characteristic in humans (Garn, Burdi,
Babler, & Stinson, 1975; George, 1930; Phelps, 1952),
which may reﬂect relative levels of ﬁrst trimester prena-
tal testosterone. Low interdigital ratio (below 1) is typi-
cal of males, and has been associated with high levels
of androgens, whereas higher ratio (1 or greater) is asso-
ciated with females and high estrogen levels (Manning,
Scutt, Wilson, & Lewis-Jones, 1998). Studies of 2D:4D
A version of this paper was presented as a poster at the meeting of the
International Academy of Sex Research, Bromont, Quebec, July 2001.
Department of Psychiatry, School of Medicine, New York University,
New York, New York.
Center for Alcohol and Addiction Studies, Brown University,
Providence, Rhode Island.
To whom correspondence should be addressed at Department of Psy-
chiatry, New York University School of Medicine, 550 1st Avenue, NV
B 20N30, New York, New York 10016; e-mail: firstname.lastname@example.org.
ratio, testosterone level, and its correlates support the as-
sumption that low 2D:4D ratio is associated with high
testosterone levels. For example, Manning and his as-
sociates have found a positive correlation between low
2D:4D and sperm count, testosterone level, male cogni-
tive skills, and athletic ability. Evidence from these stud-
ies supports the corresponding hypothesis that low testos-
terone level (high estrogen level) is positively associated
with high 2D:4D ratio as indicated by female fertility
markers (e.g. family size and waist-to-hip ratio), estrogen
levels, female cognitive skills, and associations with dis-
eases such as breast cancer (Austin, Manning, McInroy,
& Mathews, in press; Lutchmaya, Baron-Cohen, Raggatt,
Knickermeyer, & Manning, 2002; Manning, 2001, 2002;
Manning et al., 1998; Manning & Bundred, 2000;
Manning & Taylor, 2001; Manning, Trivers, Singh, &
Thornhill, 1999). Other researchers have found an asso-
ciation between low 2D:4D ratio and congenital adrenal
hyperplasia (Brown, Hines, Fane, & Breedlove, in press).
It has also been reported that 2D:4D ratio is lower in ho-
2003 Plenum Publishing Corporation