Flegr, J. (2023). Sibling Manipulation Hypothesis of Male Homosexuality. In: Shackelford, T.K. (eds) Encyclopedia of Sexual
Psychology and Behavior. Springer, Cham. https://doi.org/10.1007/978-3-031-08956-5_2307-1
1

Sibling Manipulation Hypothesis of Male
Homosexuality

Jaroslav Flegr

Department of Philosophy and History of Science, Faculty of Science,
Charles University, Prague, Czech Republic. E-mail: [email protected]

Synonyms
Fraternal manipulation hypothesis.

Definition
The sibling manipulation hypothesis of male homosexuality
postulates that older brothers, during their embryonic development,
induce changes in the maternal organism that influence the sexual
orientation of their later-born brothers. This increases the chances
of these siblings being homosexual, which in turn lessens
competition for family resources and mating partners, enhancing the
older brothers' direct fitness.

1. The Mystery of Male Homosexuality's Persistence
The emergence and long-term persistence of male homosexuality in
the majority of human populations represent one of the many
unresolved evolutionary mysteries. :omosexual men tend to have
significantly fewer offspring (Ciani, Battaglia, & Zanzotto, 2015), so
this trait should rapidly disappear from the population. :omosexual-
ity exhibits heritability, but it is possible that this trait is not
genetically transmitted from generation to generation but rather
epigenetically. Even in such a case, however, the trait would
significantly reduce the biological fitness of its carrier. =t is therefore
surprising that a gene (allele) protecting its carrier from expressing
this trait has not emerged in the human genome over time.

2. Potential Explanations for Male Homosexuality
:omosexual individuals are not only found in the majority of human
populations but also occur at a relatively high frequency within these
populations. This suggests that homosexuality could be an
adaptation that somehow enhances its carrier's inclusive fitness. This
may happen through the pleiotropic effect of the gene for
homosexuality - under certain conditions, a given allele causes the
emergence of homosexuality, while under other conditions, it does
not and instead increases the fertility or viability of its carrier (Miller,
2000). Alternatively, the allele may decrease the direct fitness of its
carriers while simultaneously increasing the fitness of their relatives,
and therefore the overall inclusive fitness of carriers of the allele for
homosexuality (Zietsch et al., 2021). The same-sex affiliation
hypothesis suggests that homosexuality might have a purpose
outside reproduction, mainly fostering enduring alliances between
men (Kirkpatrick, 2000). The bisexuality byproduct hypothesis
suggests that the roots of homosexuality could stem from bisexuality.
=t proposes that men who engage in bisexual behaviors might have a
reproductive advantage over those who exclusively display
heterosexual behaviors (Dewar, 2003). While these hypotheses
present intriguing possibilities, none have yet been conclusively
substantiated by empirical data.

3. Manipulation Hypotheses of Male Homosexuality
:owever, homosexuality might not be an adaptation, a trait
increasing its carier’s fitness. =t could be a xenoadaptation, a trait
that benefits another biological entity and is controlled by that
entity's genes. One explanation for male homosexuality based on
xenoadaptations offers the older parental (maternal) manipulation
hypothesis (Ruse, 1988; Trivers, 1974), and the second, newer, sibling
manipulation hypothesis (Flegr, 2022).
Evidence indirectly supporting these two hypothesis is found in
multiple observations that demonstrate a rising probability of
homosexuality in a woman's male offspring with each subsequent
older brother, a known phenomenon called the fraternal birth order
effect (Slater, 1962). Reinforcing this, a meta-analysis that gathered
24 samples of homosexual and heterosexual men from 18 different
studies concluded a significant increase in the likelihood of a man
being homosexual if he has older brothers (OR = 1.28) (Blanchard &
Bogaert, 1996). Another study has shown that each older brother
increases the probability of homosexuality of younger brothers by
about 33% (regardless of whether they lived together or were
separated after birth) and that only older biological brothers, not
stepbrothers (or older or younger sisters), have this effect (Bogaert,
2006). According to the parental manipulation hypothesis, the
mother programs the embryogenesis of later-born sons so that they
are more likely to be born homosexual. =n socioeconomically highly
stratified polygamous societies, only the highest-ranking men
reproduce, and those with low and middle-ranking have little chance
of reproduction. From the perspective of the mother's fitness, it is
more advantageous if younger homosexual brothers do not attempt
to reproduce and thus allow the allocation of maximum family
resources to the eldest brother.
The molecular mechanism behind the manipulation is unknown.
Generally, the maternal organism may alter a son's future sexual
preferences during embryonic development, for example, through
the effects of some hormones on the developing nervous system or
the expression of some genes in cells of the nervous system. The
mother's organism can gain information about the existence of
previous sons from the presence of antibodies against male embryo-
specific antigens. A 2017 study suggests that the maternal
immunological response to the Y-chromosome-linked protein
neuroligin probably plays a role in the development of male
homosexuality. Neuroligin is a cell adhesion molecule thought to play
an essential role in specific cell-cell interactions in brain development
(Bogaert et al., 2018).

4. Sibling Manipulation Hypothesis of Male Homosexuality
According to the sibling manipulation hypothesis, a male embryo
influences the maternal organism, possibly by inducing immunity to
male embryo-specific antigens, leading to an increased probability
that younger brothers will be born homosexual. By orchestrating
this, older brothers serve their own interests by lessening the
competition for family resources and, most significantly, mating
partners. =n doing so, they increase their own direct fitness but
compromise an essential component of their inclusive fitness – their
younger brothers’ direct fitness. Nevertheless, based on :amilton's
rule, resources invested by the older brother in his offspring (with a
relatedness coefficient of 0.5) are twice as valuable as the same
amount of resources invested in younger brothers' offspring (with a
relatedness coefficient of 0.25) (:amilton, 1964a, 1964b). Therefore,
the overall budget of manipulation is positive for manipulators. The
sibling manipulation hypothesis could explain the persistence of
homosexuality in monogamous societies without significant
socioeconomic stratification, thus offering a more universal
explanation for the fraternal birth order effect than the older paren-
tal (maternal) manipulation hypothesis.

Flegr, J. (2023). Sibling Manipulation Hypothesis of Male Homosexuality. In: Shackelford, T.K. (eds) Encyclopedia of Sexual
Psychology and Behavior. Springer, Cham. https://doi.org/10.1007/978-3-031-08956-5_2307-1
2

5. Homosexuality as a Reproductive Strategy in Highly Stratified
Societies: Biological Adaptation or Manipulation?
=n societies characterized by high socioeconomic stratification,
homosexuality could notably boost the inclusive fitness of younger
brothers. This enhancement in inclusive fitness results from a greater
number of gene copies passed onto subsequent generations via the
offspring of older brothers. This may more than compensate for the
decrease in their direct fitness due to fewer gene copies transmitted
in their own, likely fewer, offspring. Consequently, in such settings,
biological factors may predispose younger brothers to homosexual-
ity. Therefore, male homosexuality may not necessarily be the
outcome of manipulation by older brothers or mothers but could be
a strategic biological adaptation of younger brothers themselves.
With the advent of agriculture, human societies witnessed a
period of significant socioeconomic stratification (Apostolou, 2010;
Stephens, 1963). =n numerous such societies, primarily the eldest
son, the customary inheritor of the family's resources, had a
substantial opportunity for successful reproduction or securing a
high-quality partner. =n response, younger brothers were driven to
adopt alternative reproductive strategies, with homosexuality
potentially emerging as one viable approach.
=n this context, the antibodies against male embryo-specific
antigens present in the mother's blood may not represent a
manipulative tool employed by mothers or older brothers, intending
to steer the sexual orientation of younger brothers towards
homosexuality to enhance the direct fitness of older brothers.
=nstead, these antibodies might act as biological cues, not signals, for
the embryos of younger brothers, denoting the presence of older
siblings, potentially swaying them towards homosexuality.
Generally, the younger son adaptation hypothesis, with its emphasis
on cues rather than signals, offers a novel pathway towards
understanding the evolutionary and developmental mechanisms of
male homosexuality.

6. Testing the Manipulation Hypotheses of Male Homosexuality:
Parental vs Sibling Manipulation vs Younger Son Adaptation
=n the exploration of male homosexuality within the context of
socioeconomically stratified societies, three primary hypotheses
have emerged: parental manipulation, sibling manipulation, and the
younger son's adaptation. To discern the validity of these hypothe-
ses, a comprehensive investigation that integrates fields such as
human reproductive biology, embryology, genetics, immunology,
and sociobiology is paramount.
The parental manipulation hypothesis suggests that the
occurrence of homosexuality in a family enhances the mother's
biological fitness, specifically by increasing the number of
grandchildren. To test this, it would be necessary to compare the
effect of a man's homosexual orientation on his mother's fitness. On
the other hand, the sibling manipulation hypothesis posits that older
brother's manipulation increases the direct fitness of elder brothers
at the expense of younger brothers' fitness. This implies that the
presence of homosexual brothers in the family could reduce the total
number of a mother's grandchildren, thus decreasing the mother's
biological fitness. These contrasting effects present a clear distinction
that can be investigated within appropriate societal contexts.
The third hypothesis, that of the younger son's adaptation,
proposes that the transition to homosexuality may be a biologically
advantageous response to a highly stratified socioeconomic environ-
ment. =n this scenario, the average inclusive fitness of homosexual
younger brothers should surpass that of their heterosexual
counterparts. Theoretically, we can distinguish between these
scenarios by comparing the inclusive fitness of homosexual and
heterosexual younger brothers. =f the inclusive fitness of
heterosexual younger brothers is higher, this would suggest that the
shift to homosexuality is more likely an outcome of parental or sibling
manipulation.
:owever, testing these hypotheses within modern human
societies poses significant challenges. The dramatic transformation
of social environments, combined with widespread contraceptive
use which separates sex from reproduction, means that modern
humans may not be the ideal species for such investigations. This
context substantially alters the dynamics of these hypotheses and
must be thoroughly considered in future research.

Conclusion
The exploration of the evolutionary dynamics of male homosexuality
has generated a variety of intriguing hypotheses, each positing
different mechanisms for the phenomenon's persistence across
human populations. These hypotheses include parental manipula-
tion, sibling manipulation, and the younger son's adaptation, which
all offer unique perspectives, intertwining with our understanding of
human reproductive biology, embryology, genetics, immunology,
and sociobiology.
The parental and sibling manipulation hypotheses suggest a level
of direct fitness gained by the mother or elder brothers through
manipulation of the sexual orientation of the younger brother during
his embryonic development. Conversely, the younger son's
adaptation hypothesis sees homosexuality as a strategic response of
the younger brother's embryo to the probable existence of an older
brother in a highly stratified socioeconomic environment. Although
these hypotheses offer promising routes for validation, putting them
to the test within modern human societies poses significant
challenges. The rapid social transformation and the separation of sex
from reproduction necessitate careful contextual consideration.
=n this context, the sibling manipulation hypothesis offers an
interesting angle that incorporates both biological and social factors.
This hypothesis posits that male homosexuality may be a manifesta-
tion of competition between siblings for resources, particularly in the
realm of mating. :owever, this hypothesis also demands rigorous
empirical scrutiny.
Each hypothesis presents a unique perspective, highlighting that
the expression of human sexuality, including homosexuality, is likely
influenced by a complex interplay of genetic, hormonal, environmen-
tal, and social factors. =t's also crucial to acknowledge the possibility
of various subgroups of gay men with potentially distinct
biodevelopmental origins of homosexuality (Swift-Gallant, Coome,
Aitken, Monks, & VanderLaan, 2019). The maternal immunological
reaction is hypothesized to be a proximal cause of homosexuality in
only one of these subgroups. As such, future research should aim to
comprehensively integrate these factors, paving the way for a more
nuanced understanding of the complexity of male homosexuality.

Cross-References
Parental manipulation hypothesis of male homosexuality; younger
son's adaptation hypothesis of male homosexuality; xenoadapta-
tions; male homosexuality; sexuality;

Flegr, J. (2023). Sibling Manipulation Hypothesis of Male Homosexuality. In: Shackelford, T.K. (eds) Encyclopedia of Sexual
Psychology and Behavior. Springer, Cham. https://doi.org/10.1007/978-3-031-08956-5_2307-1
3

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