bailey and pillard

BIOLOGICAL INFLUENCES ON SEXUAL ORIENTATION

KEY TERMS DEFINED

• Genetic Inheritance: The transmission of genes from parents to offspring, influencing physical and behavioral traits.

• Sexual Orientation: A person’s pattern of emotional, romantic, or sexual attraction to others.

• Monozygotic (MZ) Twins: Identical twins, sharing 100% of their genetic material.

• Dizygotic (DZ) Twins: Fraternal twins, sharing approximately 50% of their genetic material.

CONTEXT

• This study is grounded in the biological approach in psychology, specifically the genetic basis of behavior.

• Historically, homosexuality was pathologized, with little biological research due to stigma.

• The study connects to the Human Relationships option, examining how biological factors influence interpersonal phenomena like sexual orientation.

THESIS STATEMENt

This essay will evaluate the extent to which genetic inheritance influences sexual orientation, using Bailey & Pillard (1991) as a central study, and will critically assess the strengths and limitations of its methodology and findings.

THEORY

• If genes influence sexual orientation, then individuals with greater genetic similarity (e.g., MZ twins) should have higher concordance rates for homosexuality.

• This supports the idea that human behavior—especially in relationships—may be biologically determined to some degree.

AIM

• To investigate whether homosexuality has a genetic basis by comparing concordance rates among MZ twins, DZ twins, and adoptive brothers.

METHOD

• Participants: Male MZ and DZ twins, and adoptive brothers, recruited via advertisements in gay publications (non-random, volunteer sampling).

• All participants were raised together to control for shared environmental effects.

• Sexual orientation and Childhood Gender Nonconformity (CGN) were assessed via questionnaires and interviews.

Variables

• Independent Variable (IV): Genetic relatedness (MZ twins, DZ twins, adoptive brothers)

• Dependent Variable (DV): Concordance of sexual orientation (whether both brothers identified as homosexual)

FINDINGS

• MZ twins: 52% concordance rate for homosexuality

• DZ twins: 22% concordance

• Adoptive brothers: 11% concordance

• Non-twin brothers (later study): 9.2% concordance

• MZ twins were more similar in CGN, although no correlation was found between CGN and adult homosexuality.

APPLICATIONS

• Suggests genetic inheritance may influence sexual orientation.

• Supports biological explanations for human behavior in interpersonal contexts, challenging past psychiatric or sociocultural models.

• Influences the nature vs. nurture debate, providing evidence for heritability.

EVALUATION

Strengths

• Provides empirical support for the role of genetics in sexual orientation.

• Inclusion of adoptive brothers as a control for environment strengthens the argument for genetic influence.

• Helped shift perspectives on homosexuality away from moral or psychiatric explanations.

Limitations

• Sampling bias:
  

  • Recruitment through gay publications may lead to ascertainment bias—participants more likely to have a gay relative, inflating concordance rates.

• Self-report data:
  

  • Sexual orientation and childhood behaviors were assessed through interviews, which are vulnerable to recall bias and social desirability.

  • Zygosity (MZ vs. DZ) was not confirmed through genetic testing, only self-reported or judged by physical similarity.

• Construct validity:
  

  • Definitions of “homosexuality” may differ across participants and cultures; the concept is complex and fluid.

• Generalizability:
  

  • Only male participants were included; findings may not apply to females or non-binary individuals.

  • Sample size limited due to the rarity of twin pairs where at least one is homosexual.

• Reductionism:
  

  • Focusing solely on genetics neglects social, cultural, and psychological influences.

  • MZ twins share more than genes—they often experience more similar environments, which could explain their greater concordance.

COUNTERARGUMENTS

• The study does not rule out environmental factors: MZ twins may be treated more similarly than DZ twins or adoptive siblings.

• High similarity in CGN among MZ twins might reflect shared upbringing, not purely shared genetics.

UNANSWERED QUESTIONS / CRITICAL THINKING

• What is the role of environmental factors, including parenting, culture, or social experiences?

• Would similar patterns of genetic influence be found among female or nonbinary participants?

• Could epigenetic or prenatal hormone factors (rather than direct gene coding) better explain these findings?

PRACTICAL USES / IMPLICATIONS

• Promotes greater societal acceptance by framing sexual orientation as partly biologically influenced.

• Challenges outdated psychiatric models that treated homosexuality as a disorder.

• Informs policy and education regarding the diversity of sexual identities.

CONCLUSION

• Bailey & Pillard (1991) provides compelling—though methodologically limited—evidence that genetic factors may influence sexual orientation.

• The greater concordance among MZ twins compared to DZ or adoptive brothers supports some heritability, but the data is not conclusive.

• The study marks an important biological contribution to understanding sexual orientation but underscores the need for more rigorous, diverse, and inclusive research.

• Ultimately, sexual orientation likely results from a complex interaction of biological, psychological, and environmental factors.