7. Same Sex Aggression- Challenge hypothesis Flashcards Preview

human evolution > 7. Same Sex Aggression- Challenge hypothesis > Flashcards

Flashcards in 7. Same Sex Aggression- Challenge hypothesis Deck (42)
Loading flashcards...

What are the costs of Testosterone?

Wingfield et al. 2001

• Interference with paternal care
• Exposure to predators
• Increase risk of injury
• Loss of fat stores
• And possibly immune system function and oncogenic (tumour casing) effects.


Why would we expect to find T in human aggression?

• the clear association between age and physical aggression  young adult males show higher levels of physical aggression, violence and homicides to members of the same sex (Archer, 2004) than do other age and sex categories.  fit the view that human male aggression is, like that of many other mammals heightened when reproductive competition is most intense

• Among birds and mammals, one widespread mechanism that increases the readiness of males to fight during phases of the life-history when reproductive opportunities are greatest is the action of testosterone on areas of the brain controlling aggressive behavior (Archer, 1988). T is enhanced at times in life when males need to compete for receptive females or the resources necessary to attract such females, i.e. in the breeding season or after sexual maturity.


How true is it that T controls violence in males in a linear fashion?

The view that testosterone secreted by males at puberty and throughout adult life facilitates human male aggression, despite its superficial appeal and its incorporation into media accounts of young men’s behavior, is at best an oversimplification.

The consensus is that there is a weak and inconsistent association between testosterone levels and aggression in adults (Archer 2006), and that the administration of testosterone to volunteers typically does not increase their aggression (O’Connor et al., 2004).


Who made the Challenge Hypothesis?

Wingfield et al, 1990


who made the argument that the challenger hypothesis could also be applied to humans?

Archer (2006) makes the argument that this can be applied to humans also.


what does the challenge hypothesis stipulate?

- there are specific context-dependent increases in testosterone levels that are associated with aggression

-T levels rise to moderate levels at the start of the breeding season and these levels support reproductive physiology and behaviour.

-During challenges to males in contexts that are relevant to reproduction, T levels rise further

-This in turn facilitates aggression in the context of territory formation, dominance disputes and mate guarding (Wingfield et al. 2000).

-Then.. when males are required to care for offspring, their T levels decrease.


Support for challenger hypothesis in Animals

Administering high levels of exogenous testosterone to several species of birds increased their mating and aggressive behaviour and supressed their paternal behaviour –Wingfield, 1984.


support for Challenger hypothesis adaptation in chimps. Background to study

Muller and Wrangham 2004

o unlike the bird species on which the hypothesis was based, chimpanzees are neither seasonal breeders nor monogamous.
o Reasoning that the hypothesis may apply in different ways to mammals, the researchers predicted that since access to receptive and fertile females was relatively uncommon for male chimpanzees, there should be maximal aggression at such times. Indeed, it is known that males are more aggressive in the presence of parous estrous females.
o Muller and Wrangham also predicted that testosterone levels would be greater for dominant than for low-status males, since dominant males were more aggressive at all times.


support for Challenger hypothesis adaptation in chimps. Predictions

Muller and Wrangham also predicted that testosterone levels would be greater for dominant than for low-status males, since dominant males were more aggressive at all times.


support for Challenger hypothesis adaptation in chimps. Findings

o Got urine samples from chimps and found support for their modified version of challenger hypothesis in a number of ways:
o 1) When parous (offspring bearing) females were in estrus, there was a significant increase in testosterone and also an increase in aggressive competition. This did not occur in the presence of nulliparous females, whom males generally find less attractive but with whom they still copulate
o 2) Also consistent with predictions, dominant males were more aggressive than low-ranking ones, and produced higher levels of testosterone


why would we expect to see the challenge hypothesis in humans males?

The range of possible mating systems, which can be linked to ecological conditions prompts the suggestion that there could be a role of testosterone in these alternative mating systems in humans.


how do humans vary from chimps and how do human males vary to each other?

Humans show neither breeding seasons nor oestrus.

They show paternal care, but the extent to which individual males are committed to paternal care is variable both according to ecological conditions and according to individual variation. There may be considerable variation between men in this and other attributes resulting from sexual selection, for example in mate choice preferences and physical aggression.


who extended the challenge hypothesis to incorporate individual differences?

Archer 2006


what 6 predictions did Archer make about the challenge hypothesis in males?

1) there is no increase in aggression at puberty
2) Men respond to sexual arousal/ threat with increased T
3) The T response to challenge increases aggression
4) T levels lower in paternal men
5) Aggressive dominance is correlated with T
6) T is associated with alternative life history strategies


how can Archer's 4 and 5th hypothesis be extended into the 6th?

They highlight that there are adaptive costs, as well as benefits of high T levels. Thus there is a trade-off between mating effort (supported by high T levels) which involves the cost of high T levels but may lead to enhanced fitness benefits and parental effort (supported by lower T levels), which involves a lower cost and lower fitness benefit).

Testosterone levels vary among human males, and this variation may reflect a relatively enduring preference for one of two life-history strategies


what would be the two life-history strategies in males.

The first (high parental investment) involves an emphasis on long-term commitment to one mate and paternal care, accompanied by low mating effort, whereas the second (low parental investment) involves low commitment to one mate and no paternal care, accompanied by high mating effort.


what are we unlikely to see in life-history strategies in human males?

. It is unlikely that these will be absolute differences in humans, as they are in the males of some other species. Rather there will be relative differences in the degree to which males devote time and energy to mating and parental effort. These individual differences will be associated with differences in testosterone levels, and they represent long-term differences in life history strategies.


evidence for hypothesis 1) No increase in aggression at puberty

• Halpern et al 1994- reported 3 year longitudinal study of 100 adolescent boys, whose initial mean age was 13.3 years. Measures included plasma hormone levels and a self-report rating of physical and verbal aggression, the OMAI, which showed high correlations with testosterone among 15–17 year old boys. Although, as expected, testosterone levels greatly increased when the boys went through puberty, no change in direct aggression was found, and there was no correlation between testosterone and aggression. In the same sample, there was a clear increase in sexual activity as the boys went through puberty.

• Results supported by many other studies, as a meta-analysis by Archer, 2004 showed that sex differences in direct aggression did not show a pronounced increase at puberty (self reports, peer reports)


why was hypothesis 1 that aggression would not increase at puberty made?

challenger hypothesis makes clear that there are two types of testosterone-behavior relationships, the first is to support courtship and mating, and the second is to facilitate competitive behavior in relevant circumstances. The first occurs in birds throughout the breeding season in seasonal breeders, and from sexual maturation onwards in non-seasonal breeders. The second involves increases in testosterone that are facilitated by reproductive competition. We would therefore expect no testosterone-induced increase in aggression at puberty in human males, in contrast to the facilitation of sexual interest by testosterone at this time.


evidence for 2.1 men respond to sexual arousal with incresed T

• Roney et al. 2003- measuring men’s testosterone levels as a consequence of brief interactions with a potential mate- exposed young men to a brief friendly conversation with young women or (as a control) young men. Although they found a significant increase in testosterone levels after interacting with the woman there was also a non-significant increase among the participants who interacted with the young man rendering the difference between the two conditions non-significant for this small sample However, from the challenge hypothesis we might predict an increase in testosterone levels when meeting a young man for the first time, as he might represent a competitor. The study did therefore provide clear evidence that interacting with a potential mate raised young men’s testosterone levels. Further, it was found that this increase was largely restricted to those men who said that they had had a recent sexual experience. There was also a strong correlation between testosterone levels and the woman confederate’s ratings of the male participant’s display behaviour towards her.

Hellhammer et al. (1985) examined salivary testosterone levels among 20 young men before, during, and after exposure to erotic, sexual, aggressive, stressful, and neutral films. Testosterone levels showed an increase 15 min after the beginning of the erotic and sexual films. No sig increases for other types of film.


evidence for 2.2 men respond to threat with increased T

Archer 2006- conducted a meta-analysis on situations where men were in competition either lab studies or in real life sports. 23 samples found that there was an increase in T during the competition irrespective of whether it involved a contrived laboratory task or a sport competition. We can therefore conclude from the evidence on competitions that there is a small anticipatory rise in testosterone before sport competitions and that overall there is evidence of a small increase from before to after a competitive situation.

Cohen et al. (1996) reported an experimental study of students from the North and the South of the United States, in which they were insulted or not insulted, and their subsequent behaviour and physiology studied. There were greater increases in testosterone from before to after the insult among insulted southerners than in the other three conditions (insulted northerners and both non-insulted conditions), and also much higher cortisol levels. These physiological changes paralleled several psychological changes. These included greater immediate expression of anger, being more likely to complete a questionnaire involving sexual jealousy with violence, greater willingness to challenge a larger individual, and being more domineering to a smaller person. This study indicates that young men who are from a culture where honour is important do respond to an insult- a challenge to honour- with an increase in T and with more aggressive, domineering, behaviour. Whether the testosterone surge plays any causal role cannot be inferred from the study


evidence for 3. T response to challenge increases agression

• Berman et al. 1993- lab study using competitive situation designed to assess aggression found that higher plasma T levels predicted higher aggression levels- used a a competitive reaction time task in which participants administered shocks to a supposed confederate. Positive correlation between initial T levels and subsequent aggression but also evidence that high T participants were more motivated in the task than were low T participants implicating individual difference.


evidence against 3. The T response to challenge increases aggression but issue with this also

Archer 2006- meta analysis on humans injected with T and effects and concluded that there is no consistent evidence for increases in measures associated with aggression following the artificial raises of testosterone levels, yet argues that this may be because many of these studies are not designed to specifically test predictions from the challenge hypothesis, for example that testosterone may increase competitiveeness between males, especially where some status-related outcome is at stake. Suggest that Men whose disposition makes them prone to be directly competitive with other men may react more strongly when their testosterone levels are artificially raised.


Why was it predicted that 4. T levels are lower among paternal men in animals

• There is evidence for a consistent pattern of hormonal changes in paternal birds and mammals (at around the time of birth, which does not occur in males of non-paternal species (Wingfield et al., 1990, 2000)


evidence for 4. T levels are lower among paternal men

• Does this exist in humans- Storey et al (2000) measured salivary T levels from 24 couples at one of 4 times before and birth- T levels were 33% lower in samples taken in the early postnatal than in the late prenatal phase in men. Additionally, men who were more responsive to auditory, visual and olfactory cues from newborn infants had lower testosterone levels, or larger decreases from before.
• Berg and Wynne-Edwards (2001) also found that testosterone levels were lower in the fathers than in 14 age-matched controls.


what can we say overall about T levels in paternal men?

• Although the evidence is based on few studies involving small samples, it seems clear that fatherhood is associated with lowered testosterone levels. There is also evidence of lowered testosterone levels, and other hormonal changes, similar to those found in other paternal species, at the time of birth and infant care. This is also the pattern predicted from the challenge hypothesis.


who argued similarities in hormonal changes in maternity and paternity

• Wynne Edwards (2001) has argued that the hormonal changes associated with paternity involve homologous neuroendocrine circuits to those involved in changes associated with maternity.


why would we expect hypothesis 5. Aggressive dominance is correlated with T levels to be true in humans

• In non-humans dominance based least initially on physically-based contests- probably also the case in most pre-state human societies and in modern world societies where there is no effective rule of law- thus predict that aggression based dominance will be associated with higher T levels- but other forms of dominance, such as those achieved through social networks or occupational skills, will not necessarily relate to testosterone levels in this way


evidence for Aggressive dominance being related to T

• Studies involving prisoners generally reported higher testosterone levels among those classed as aggressive or violent than in non-violent prisoners
• Archer, 2006- for the majority of studies there is a consistent association between testosterone levels and various measures of dominance.
• Schaal et al. (1996) distinguished between having a history of fights, toughness, and leadership (assessed by others) in a longitudinal sample of boys aged 6–12. Those with a history of many fights had lower testosterone levels than those with fewer fights. At age 13, it was the boys who were rated as tough by unfamiliar peers who had high testosterone levels. A combination of high ratings on toughness and on leadership predicted the highest levels of testosterone in this sample.
• van Honk et al. (1999) found that testosterone levels measured 6 h previously were more closely associated with attending to angry faces on a Stroop task than were levels taken 4 h before or concurrently. They suggested that this is another manifestation of the association between physically-based dominance and testosterone levels, since aggressively dominant people are likely to perceive an angry face as a challenge and stare at it rather than looking away. These associations were found in both sexes and were stronger for women than for men.


6) testosterone is associated with alternative life history strategies - this in itself leads to 4 further predictions what are these?

1) testosterone and the allocation of mating versus parental effort (pursuit of extra-pair/ short term relationships as opposed to stability of one relationship and parental effort. 2) T levels and personality characteristics that may predispose individuals to enter such different life courses 3) associations between T and antisocial activities that would tend to be associated with a shorter-term, higher-risk, reproductive strategy. 4) there are studies relevant to the issue of whether the associations found in adulthood are likely to be in place early in life, and hence represent enduring specialized life history strategies