New Evidence of Genetic Factors Influencing Sexual Orientation in Men: Female Fecundity Increase in the Maternal Line Katelyn, Bo, and Hana

Introduction: Previous Research Hamer, et all(1993) studying homosexual brothers (increased rate of homosexuality in maternal lines)  hypothesized a genetic factor located on X-chromosome Role of maternal immune reactions  Immune reaction against the H- Y antigen acting against male differentiation of brain (prenatal cause)

Introduction: Previous Research King et al. (2005) showed that homosexuals have larger families compared with heterosexuals irrespective of the paternal line and maternal line

Introduction: Darwinian Paradox Natural selection should progressively eliminate factors that reduce individual fecundity and fitness Homosexuals reproduce significantly less than heterosexuals Wilson (1975)-Kin selection hypothesis  Not supported Ciani, et all. (2004): sexually antagonistic genetic model that promotes fecundity in females and homosexual orientation in males

Methods: Sampling 250 Probands 18 years or older, male, and not adopted 98 Heterosexual & 152 Homosexual Beaches, Resorts, Bars, Clubs, Universities, & Gyms From all over Italy 90% homosexuals & 58% heterosexuals unmarried

Methods: Questionnaire Questionnaire Data Approached during times of Inactivity 10 Min to complete

Methods: Reliability Same questions as the questionnaire in the 2004 study Questions tested in a pre-test Researches explained questionnaire & were available to clarify doubts

Biographical Information Age Region of Birth Region of Residence Academic Degree Profession Marital Status Birth Order

Sexuality: Kinsey Scale Homosexual=0&1 Heterosexual=2-6

Methods: Measuring Fecundity Grandparents Uncles/Aunts Brothers/Sisters Cousins Half asked about maternal line & half asked about paternal line

Fecundity Results Heterosexuals had higher fecundity than homosexuals Increase in fecundity of the maternal line in homosexuals relative to heterosexuals (mothers, aunts, and cumulative) No significant differences between homosexuals and heterosexuals in the paternal line EXCEPT paternal uncles (higher fecundity in heterosexuals, opposite of the trend in the maternal line)

Results – Table 2

Birth Order Results Homosexuals had more older brothers than older sisters Homosexuals had more older brothers than heterosexuals No significant differences in younger brothers or sisters

Discussion Increased fecundity in the maternal line means that homosexuality is correlated with a positive Darwinian benefit Theoretical model: Sexually antagonistic model. It is bad for the males that have the gene but good for females who are carriers Proximate mechanism? My idea: The penis hypothesis

Paternal Fecundity Only one significant difference in the paternal line comparison (and its in the opposite direction) This shows that the homosexuality gene is only correlated with higher fecundity in the maternal line Because fecundity is affected by modern technology (ex. birth control), using paternal fecundity helps alleviate concerns about the EEA

Interesting Data Things not explained well by the fecundity hypothesis: The birth order effect Paternal uncle data They acknowledge a multi-locus model is probably at work; there are many factors at work and fecundity increase is only one of the benefits

Confounds Sampling methods – heterosexual sampling population may not reflect actual heterosexual population (in clubs, not married, lower fecundity) Kinsey Scale – Bisexuality categorized as homosexual Both of these should adjust the data against their hypothesis

King et al (2008) Large samples could have lead to overrepresentation of homosexuals due to small population sampling bias  Larger families are more likely to have a homosexual child Didn’t control for “probability of sharing the X- chromosome”  Brothers and sisters were placed in the paternal and maternal line (brothers do not get their X chromosome from the paternal line => which can lead to an overrepresentation of fecundity in the paternal line)

Future Studies Replication in other cultures Is bisexuality in females the result of being carriers of the homosexual gene or just a cultural thing?