17. Human Sexual Behavior III & Aggression I

TL;DR

The most supported evolutionary explanation for homosexuality is the 'helper at the nest' model, where individuals not reproducing directly boost the reproductive success of both their siblings. Separately, averaging many human faces into a composite consistently makes them look more attractive, revealing why facial symmetry signals mate quality.

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 female-female competition happen in monogamous species?

In pair-bonding, monogamous species, a male who believes he is the father will invest heavily in raising the offspring, making a good, confident, convincingly maternal-acting partner highly valuable. This creates intense competition among females for access to those males. The resulting selection pressures, seen in New World monkeys and many bird species, can make females more aggressive, larger in body size, and more pronounced in secondary sexual characteristics than males, reversing the usual pattern.

Q: What are the three evolutionary theories of homosexuality?

The lecture presents three: the heterozygotic-vigor argument, where a homozygotic gene form lowers reproductive success while the heterozygotic form gives a compensating advantage; the gender-dependent genetic argument, where a trait is maladaptive in one gender but adaptive in the other; and the 'helper at the nest' model, where non-reproducing individuals spend resources helping their siblings reproduce. All three assume genes influence sexual orientation to at least some degree.

Q: Which theory of homosexuality has the most support?

The 'helper at the nest' kin-selection model has the most support. It proposes that individuals not passing on their own genes directly instead expend resources helping their siblings. It differs from the gender-dependent model by predicting that both sisters and brothers of gay men should show increased reproductive rates, and that broader pattern is mostly what has been observed in the literature, supporting this kin-selection argument.

Q: How common is homosexuality across human cultures?

Best estimates place the rate at 5% to 20% in every human culture ever looked at. This universality is precisely what makes it an evolutionary puzzle for zoologists and evolutionary biologists, because until relatively recent times gay men passed on far fewer copies of their genes on average across virtually every culture. The trait's persistence despite this apparent reproductive cost is what the three theories attempt to explain.

Q: Why do composite faces look more attractive?

When many individual faces are merged into a single composite, the result is perceived as more attractive than the individuals it came from. The more faces that go into the composite, the more attractive people rate it on average. This effect relates to facial symmetry, since averaging cancels out asymmetries and idiosyncrasies. People can detect subtle differences between composites, such as 50 faces versus 25, though not necessarily consciously.

Q: How was the composite-face attractiveness effect discovered?

It was discovered accidentally in the 19th century by a famous criminologist who believed in the nonsense theory that criminals had identifiable facial characteristics. He gathered photographs of many criminals and used an early photo-overlapping technique to create a composite 'archetypal criminal face.' Instead of looking criminal, the composite struck everyone as unusually good-looking, revealing that averaging faces makes them appear more attractive.

Q: What is the gender-dependent genetic theory's prediction?

The gender-dependent genetic argument holds that a genetically influenced trait is maladaptive and reduces reproductive success when expressed in one gender but is highly adaptive and increases reproductive success in the other. As long as the benefit in one gender outweighs the detriment in the other, it is selected for. Its specific prediction is that sisters of gay men should have higher-than-average reproductive rates, a pattern that is present in the literature.

Q: How does the heterozygotic-vigor theory apply to homosexuality?

The heterozygotic-vigor argument borrows from genetics examples like sickle-cell anemia, where the full homozygotic form is a serious disorder but the partial heterozygotic form confers malaria resistance. Applied metaphorically, it suggests the homozygotic form of some relevant gene produces a phenotype lowering reproductive success, while the heterozygotic form carries an advantage large enough to outweigh the one-fourth of relatives with the homozygotic version. There is very little evidence for this model.

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.