17. Human Sexual Behavior III & Aggression I

TL;DR

Homosexuality may persist through the “helper at the nest” model, in which non-reproducing individuals help siblings reproduce and thereby support shared genes. The lecture compares this explanation with heterozygotic vigor and a gender-dependent genetic theory, while noting that all three assume some genetic influence on sexual orientation. It also explains female competition in monogamous species and why composite faces appear attractive. Read on to compare the evidence and predictions.

Transcript

Stanford University. --started. See some exam-related announcements. First off, there are the statistics, mean and median, which, in terms of numbers and what they mean-- what they mean in my book is that people are doing a good job in here. So, good going. Also having done an amazing job are the TAs, who, other than coming to class here, have not... Read More

Key Insights

• Female-female competition dominates in pair-bonding, monogamous species like New World monkeys and many birds, where females compete for males who will help raise offspring, sometimes making females larger and more aggressive than males.
• Homosexuality appears at an estimated 5% to 20% rate in every human culture ever studied, creating an evolutionary puzzle because gay men historically passed on far fewer copies of their genes on average.
• The heterozygotic-vigor theory argues a homozygotic gene form lowers reproductive success while a heterozygotic form gives a large enough advantage to offset it, mirroring sickle-cell anemia, but has very little supporting evidence.
• The gender-dependent genetic theory holds a trait is maladaptive in one gender but adaptive in the other; it predicts sisters of gay men have higher-than-average reproductive rates, which the literature supports.
• The 'helper at the nest' model proposes non-reproducing individuals spend resources helping siblings; unlike the gender-dependent model, it predicts both sisters and brothers show increased reproductive rates, which is mostly what is observed.
• All three evolutionary theories of homosexuality depend on the still-unsettled assumption that genes have at least some influence on sexual orientation, an assumption complicated by prenatal endocrine evidence and unreplicated twin studies.
• Averaging faces into a composite makes them appear more attractive, and the more faces combined, the more attractive the result, a pattern people can detect even between 25-face and 50-face composites without conscious awareness.
• The composite-face discovery came accidentally from a 19th-century criminologist who overlapped photos of criminals expecting an archetypal 'criminal face' but instead produced an unusually good-looking one.

Questions & Answers

Q: Why does homosexuality persist according to evolutionary theory?

The lecture says the “helper at the nest” model has the most support among three proposed explanations. It argues that people who do not reproduce directly may devote resources to helping siblings reproduce, supporting the transmission of shared genes. Its prediction that both sisters and brothers of gay men should have increased reproductive rates mostly matches the reported literature.

Q: What are the three evolutionary theories for homosexuality?

The three theories are heterozygotic vigor, a gender-dependent genetic trait, and the “helper at the nest” model. Respectively, they propose an advantage from a heterozygotic gene form, a reproductive benefit when the trait appears in one gender, or assistance given to reproducing siblings. All depend on the unsettled assumption that genes influence sexual orientation at least somewhat.

Q: How common is homosexuality across human cultures?

The lecture gives a best-estimate range of 5% to 20% in every human culture studied. Its universality creates an evolutionary puzzle because, until relatively recent times, gay men passed on far fewer copies of their genes on average across virtually every culture.

Q: What evidence supports the “helper at the nest” model?

The model predicts increased reproductive rates among both sisters and brothers of gay men because either can benefit from a sibling’s help. The lecture says this broader pattern is mostly what researchers observe, giving the model more support than the gender-dependent explanation.

Q: What does the gender-dependent genetic theory predict?

This theory proposes that a genetically influenced trait can reduce reproductive success in one gender while increasing it enough in the other to persist. It specifically predicts that sisters of gay men should reproduce at higher-than-average rates, a pattern the lecture says appears in the literature.

Q: How does heterozygotic vigor attempt to explain homosexuality?

The heterozygotic-vigor theory proposes that a homozygotic form lowers reproductive success while a heterozygotic form provides a compensating advantage. The lecture compares this logic with sickle-cell anemia, whose partial heterozygotic form provides resistance to malaria and other diseases. It says there is very little evidence supporting this explanation for homosexuality.

Q: Why does female-female competition occur in monogamous species?

In pair-bonding, monogamous species, a male who believes he is the father may invest heavily in raising offspring, making a capable caregiving partner valuable. Females therefore compete for access to such males. In New World monkeys and many bird species, this pressure can make females more aggressive, larger, and more pronounced in secondary sexual characteristics than males.

Q: Why do composite faces look more attractive?

Averaging multiple faces cancels out asymmetries and individual irregularities, producing a more symmetrical composite that people generally find attractive. The more faces included, the more attractive the composite tends to appear. People can even detect differences between composites made from 25 and 50 faces without necessarily being consciously aware of them.

Summary

This video discusses various topics related to sexual behavior and the evolution of attraction in different species. It covers concepts such as the role of symmetry and secondary sexual characteristics in mate selection, the handicap principle, and the importance of homogamy in human relationships.

Questions & Answers

Q: What information does symmetry convey in terms of attractiveness?

Symmetry is considered attractive because it is seen as a marker of health. It indicates that an individual has good genes and is free from developmental complications or health issues. The preference for symmetrical faces is observed in humans, as well as in various animal species.

Q: How do secondary sexual characteristics play a role in mate selection?

Secondary sexual characteristics, such as large antlers or colorful feathers, are often exaggerated in males and are seen as attractive by females. These characteristics serve as signals of good health and reproductive fitness. They indicate that the male has enough energy and resources to invest in these characteristics, making him an attractive mate.

Q: What is the handicap principle?

The handicap principle, proposed by Israeli zoologist Zahavi, suggests that exaggerated secondary sexual characteristics serve as indicators of good genes and overall health. According to this principle, the larger and more extreme the characteristic, the more it signals the individual's ability to survive and thrive despite the energy cost. This principle is based on the idea that only genetically superior individuals can afford to invest in costly traits and still have enough resources to reproduce successfully.

Q: What are some explanations for the evolution of homosexuality?

There are three theories that have been proposed to explain the evolution of homosexuality. The first is the heterozygotic-vigor argument, which suggests that individuals with a mix of certain genes may have advantages in some aspects of survival and reproduction. The second is the gender-dependent genetic argument, where a trait may be maladaptive in one gender but highly adaptive in the other. The third is the "helper at the nest" model, which suggests that individuals who do not pass on their own genes directly may help their siblings in raising offspring, contributing to overall genetic success. However, the genetic component of sexual orientation is still not well understood, and further research is needed.

Q: What are some characteristics that are universally attractive in mate selection?

In addition to symmetry and secondary sexual characteristics, there are certain characteristics that are universally attractive in mate selection. These include facial features associated with health, such as clear skin and good muscle tone. Waist-hip ratio, where a ratio skewed towards larger hips is seen as attractive in females, is also considered universally attractive. Furthermore, individuals tend to be attracted to others who are similar to them in terms of religion, age, race, socioeconomic status, and political views.

Takeaways

This video highlights the various factors that play a role in mate selection across different species, including humans. It emphasizes the importance of physical traits such as symmetry and secondary sexual characteristics in attracting mates. The handicap principle explains the role of exaggerated characteristics as indicators of good health and genetic fitness. Additionally, mate selection often involves a preference for partners who are similar to oneself in terms of various personal traits. Understanding these factors can provide insights into the evolution of sexual behavior and attraction.

Summary & Key Takeaways

• The lecture completes the evolution of sexual behavior by examining female-female competition, which predominates in monogamous, pair-bonding species. Because a helpful, committed male is valuable, females compete intensely for access to males, producing selection pressures where females can be larger, more aggressive, and more ornamented than males.

• Three theories attempt to explain why homosexuality persists despite historically reducing gene transmission: heterozygotic vigor (little evidence), a gender-dependent trait adaptive in one sex (sisters of gay men reproduce more), and the 'helper at the nest' kin-selection model, which best fits data showing both sisters and brothers reproduce more.

• Attractiveness research shows facial symmetry matters, discovered accidentally when a 19th-century criminologist overlapped criminals' photos to find a 'criminal face' but instead created a good-looking composite. Averaging faces reliably increases perceived attractiveness, and more faces yield greater appeal, with people detecting subtle differences unconsciously.