Chapter 14 : Social Behaviour
Why be social?
The best insight into this question is comparative study of other species. Sociality (the tendency for an animal to associate with
other members of its species) has a particular distribution across the animal kingdom. Animals are social if it pays them to be social, and
more solitary if the cost of being social outweighs the benefits. Because selection is not efficient at crafting adaptations at the level of
group, the evolution of social or solitary life depends on the benefits/costs to the individual.
Costs and Benefits of Sociality
− Increased competition for resources.
− Groups are more conspicuous to predators.
− Groups are better at finding resources.
− Less vulnerable to predators (more alert senses, groups can be repelling to some predators)
What works depends on the circumstances: some species may have many predators, especially if they are small. Larger species
have fewer predators. For example, the Orangutans – they are the onlyAsian great ape and are one of the only solitary primates. They are
very big and live very high up in trees, making them quite protected against predators. Some are attracted to a common resource which is
not food – baboons congregate at sleeping cliffs where they are relatively safe from attack. Or huddling together to keep warm
(conversely, this can increase vulnerability to disease).
For some species, sharing food might be costly, some may suffer little when feeding as a group; sometimes food is easy to find,
sometimes it's advantageous to forage with others who share knowledge about where the best current feeding sites are. Patch size (how
food is distributed) is a key aspect of how sociality affects feeding efficiency (this is not an absolute concept). For example, a
grasshopper, to a monkey, is a small patch (small amount of food) and it won't want to share, but to a tiny mite, it is a large patch and loses
little if other mites join it.
Animals whose food comes in large, sharable patches often benefit by being social. Such as a fruiting tree in the rainforest,
where fruiting trees bloom without coordination, and having associates to help locate these sources helps. Similarly, some predators hunt
socially, such as wolves, to bring down larger and more bountiful prey.
In conclusion, sociality is an evolved adaptation adjusted to the threats and opportunities posed by the animal's environment.
Selection Pressures Favouring Human Sociality
There are no confident answers to this, as they originated from the distant past and are subject to debate. The factors discussed
here are the more likely probabilities.
The most discussed idea is that our ancestors inhabited more open, less forested country than other great apes, and spent more
time on the ground, exposed to predators. This is supported by the digestive remains of possible human ancestors such as the
Australopithecus. With open habitats come herds of grazing animals, which may have been prime prey and shows a dietary shift to
dependence on meat. However, the arguments against this are that not all savanna-dwelling carnivores are social (the cheetah is a solitary
hunter), and not all anthropologists agree that our ancestors were especially carnivorous.
Our closest living relatives, African apes (chimpanzees and gorillas), are social. However, none of the standard reasons for
sociality can be well applied to either of theAfrican apes. Why? They are not subject to much predation. What about resources?
For gorillas, feeding competition is slight (they primarily eat leaf material, which is abundant). There seems to be little cost of
associating with others, but little benefit as well.
Chimps mostly eat fruits, and sometimes leaves. Chimps are big, therefor this creates sufficient food competition for sociality.
Chimps live in “communities,” but only small foraging parties are sent out (size depending on the size of currently exploited resource).
Gorillas never hunt, but sometimes male chimps hunt antelope or smaller primates cooperatively (though only 1% of their diet is made up
The predominant view is that chimps and gorillas are social as a defence against threats posed by other members of their same
species. Male gorillas kill infants of the females with whom they have not mated. This leads to females staying with powerful and
protective males. Gorilla groups invariably contain more females than males. In chimps the hostilities are between communities. Males
sometimes invade and kill infants of unfamiliar females, and with sufficient numbers, some will even kill males in neighbouring
communities. This apparently favours sociality, even over decreased foraging efficiency.
All of these described reasons for sociality in apes could be applied to our ancestors, either any combination of them or all at the
Social Life in the Environment of EvolutionaryAdaptedness (EEA)
Anthropologists often look to contemporary hunter-gatherer societies as a source of insight about life in the EEA (see textbook
glossary), because they resemble our ancestors in one key respect: they must forage for their food, and so face very similar ecological
constraints as our ancestors did. Lack of cultivated crops and domesticated animals place a very low limit on the size of their
communities, usually seminomadic bands of 10-30 people. These bands make up communities of a few hundred people. Members shift
from one band to another if opportunities seem better elsewhere, so everyone typically knows each other, and the network of kinship is dense. Alarge proportion of a person's acquaintances would be genetically related to them.
Due to these observations, presumably these essential social features existed with our ancestors as well. Biological kinship and
familiarity are important from an evolutionary perspective because they form the two possible bases for altruism: kin selection and
reciprocity (see chapters 7 and 13).
The Role of Kinship in Human Social Behaviour
The social behaviour of a species evolves in such a way that in each distinct behaviour-evoking situation the individual will seem
to value his neighbour's fitness against his own according to the coefficients of relationship appropriate to that situation (Hamilton,
1964). (This is kin-selection theory)
Genes can spread by helping the immediate bearer reproduce, but they can also be spread by inducing altruism aimed at other
bearers of the same genes (see chapter 13). This theory has been tested amongst hundreds of species and the vast bulk of the data support
it. In humans, kinship is fundamentally important in shaping people's interactions in all kinds of societies, and traditional human societies
are organized around kinship.
Kinship and Association
Group membership often follows along lines of kinship. In the Yanomamo tribe of South America, over 13 villages with a mean
population of 102 residents per village, the average person was related to 79% of their fellow villagers. Each of those relatives shared
12% of their genes, which makes them as related as much as first cousins are.
As the village grows, average relatedness decreases, and tensions and fights grow in parallel. This eventually splits the village
into two, splitting along lines of relatedness where close relatives remain together and the new villages have higher relatedness. This
pattern holds true for modern-day voluntary associations, such as an Amish church in Pennsylvania that split into two churches, with
relatedness being higher in each new church.
Kinship and Cooperation
Kin shapes not only who we chose as neighbours, but how we behave towards them. Kin are more likely to help each other than
non-kin. In studied tribes, helping with clearing and planting land makes a difference in food production. Studied examples: a member is
more likely to be genetically related to the person that they are helping; men who shared more genes with other tribe members were able
to recruit more individuals for help with labour, in turn keeping more acres in production; food seems to be systematically shared with kin
in both foraging and food-producing societies. In both industrial and traditional societies, people systematically help their kin.
In modern industrialized societies where we often live far from kin, a Los Angeles study showed that genetically close relatives
are more likely to help than distant ones, and the greater amount of help, the more likely it was to come from kin. According to
Hamilton's rule (see chapter 13), altruism is favoured when rb > c, or when altruism can be given easily and the receiver gains the highest
fitness benefit. In the same study this held true, where more help was given to higher reproductive potential individuals (young adults),
more help was given to nieces and nephews by women who were childless.
Since adoption represents large commitment of help, it makes sense that studies show strong kin bias with this. In a study by Silk
in Oceania, of 1,613 adoptions, 1,517 (94%) were by relatives (up to 3 cousin, see table 14-1, p 331). Adoptions were concentrated
where r = 0.25 (niece/nephew) with over 1,000. Brothers and sisters had little adoptions due to the fact that they are usually too close in
age to adopt their own.
Kinship and Aggression
Kin selection predicts decreased selfishness amongst kin. Martin Daly and Margo Wilson (1988) first to realize that our conflict
resolution tactics can be shaped by selection. Homicide is the most selfish act of conflict resolution. Does genetic kinship reduce lethal
A complication is that killing requires access. Since it's impossible to define a “victim pool” for an homicide, you can't simply
compare the amount of possible relatives versus non-relatives killed. What are looked at instead are domestic homicides. Were relatives
the victims more often in domestic homicides? Spouses are not blood relatives, so kin selection is not expected to influence the level of
violence against spouses.
The study done by Daly and Margo took Detroit 1970 census data of persons 14 or older (<1% of homicides were done by <14yr
people). Simplified answers following. For the math, see p 331/332. If kin selection has no effect, average spouse homicide rate = 20% of
domestic homicides. True data: Every type of nonrelative is killed more often than expected by chance. 65/98 domestic homicide victims
were spouses, spouses were killed 3.32 times more often, and kin were killed 0.29 times as often than if kin had no effect on aggression
deterrence. Calculated together, coresident non-kin are 11.35 times more likely to be killed than kin. These numbers underestimate the
true results, as some “offspring” were not blood related to the parents/step-parents. Being linked by marriage, such as step-child or
brother-in-law or spouse maytho lthtle to reduce violent conflict. Biological kinship seems to inhibit lethal violence.
Throughout the 9 -13 century, documents show that kin were more likely to support, form alliances with, have less conflict
with, and be less prone to kill one another. Some would even band together to kill non-kin (shown with vikings, Icelandic families,
English royalty). Same thing occurred with the previously mentioned Yanomamo tribe, when conflict arose and each side were more
related to each other than their opponents and bystanders.
Because the data presented is mostly anthropological, social psychologists often try to assess underlying attitudes and
inclinations that shape social behaviour. Very few social psychologists take interest in studying kinship. A social psychologist E.
Burnstein et. Al (1988) tested a range of predictions from kin-selection theory with American and Japanese undergraduates. They were to imagine scenarios that include several individuals, from varying levels of relatives or acquaintances, in scenarios ranging from dire
(somebody's life on the line) to casual (running an errand), and they were asked which one person they would help in each scenario.
Results: consistent with other studies, they were more inclined to help kin than non-kin, and more inclined when the benefit is large (dire
circumstances) see fig. 14-1. This shows that:
a) Kinship is a dominant factor in decisions
b) Kinship exerts stronger influence in critical situations (when fitness consequences are large)
In the same study, participants asked to imagine a world with high infant morality, disease, famine and short life spans. People in
need were of same relational value (such as 10 y.o. Nephew [r=0.25] and 75 y.o. Grandfather [r=0.25]). Results show that students were
less likely to help both younger and older individuals, with help peaking at 10 y.o (close to sexual maturity) (see fig. 14-2). This parallels
what evolutionists call the reproductive value curve, where help is more distributed towards the individual that has the highest chance of
reproductive success. When responding to scenarios that did not include high famine, stress or infant mortality, students showed no
tendency to withhold help from very young relatives.
In a similar study by Petrinovich et. al. (1993), a different hypothetical situation was posed where a street car was out of control,
and you had to flip a switch to change the direction and save the car. Flipping it left would kill 5 strangers on the tracks, flipping it right
would kill your brother/sister on the tracks. The majority chose to save their sibling.
In a recent, very similar study to the one above by the same researchers, they added variation on the left side between 1-15
strangers, and from sibling to cousin on the right side. 70% chose to save their sibling, and 60% saved their cousin, even at the cost of the
lives of 15 other people.
Proximate Mediation of Kin Altruism
Although many psychologists study the causes of altruism, surprisingly “kinship” is not a proposed relevant factor. Studied
factors are more like “emotional closeness,” “empathetic concern” and “oneness,” and these factors are measurable and show how close
two people feel their fates are connected.
Two studies (Korchmaros & Kenny, 2001; Kruger, in press) found similar results when comparing the psychological mediators of
altruism (PMAs) (listed above) against relatedness: both PMAs AND genetic closeness predicted altruism better than chance. PMAs
couldn't explain the pattern of helping by themselves; genetic closeness added significant prediction power. Have yet to discover why
variables like oneness predispose altruism and how relatedness does.
Kinship and Health
Flinn and England (1995) measured cortisol levels of children growing up in a rural village in