The twin’s method can provide an effective way to estimate the effect of phylogenetic
and ontogenetic factors on behavior. However, when reviewing the literature, it was
noticed that in Brazil the studies with twins are recent and no research has yet been
found on any aspect of sexuality using this methodology. Thus, in this exploratory
study, the objective was to investigate the sexual orientation of twins in a sample in the
northern region of the country. The twins responded, online and in person, to an
inventory of zygosity, a socioeconomic questionnaire and Klein's sexual orientation
grid. A total of 140 adult twins participated in this study, distributed in 42 pairs of
monozygotic, 18 pairs of dizygotic and 20 individuals whose cotwins did not respond to
the instruments. The participants' ages ranged from 18 to 37 years, with a mean age of
24.6 years. 69.3% of the twins were classified as monozygotic and 30.7% as dizygotic.
Regarding sexual orientation, 64.9% of monozygotic individuals identified themselves
as heterosexual only and 8.2% as gay only. While 65.1% of the dizygotic identified
themselves as heterosexual only and 9.3% as gay only. The pairs of monozygotic twins
presented significant intraclass correlation values for the three variables of sexual
orientation investigated: "sexual attraction", "sexual behavior" and "self-identity",
analyzed in three periods (past, present and ideal). Showing a concordance rate that
varied between 28% and 42%. However, among the dizygotic twin pairs there was no
correlation. The results indicated that aspects related to sexual orientation are not
permanent, demonstrating that sexual orientation can be a dynamic process. These
results were not sufficient to provide evidence that aspects related to sexual orientation
are influenced by both genetic factors and environmental variables. For this, a larger
number of participants and statistical analyzes of heritability would be necessary. Based
on this study, a voluntary registry of twins was created in this region, which may
contribute to novel studies, with a greater range of samples, mainly dizygotic, and more
representative of non-heterosexual sexual orientations, allowing new discussions to be
developed for a greater understanding of sexual orientation.
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