General Lecture Notes for Second Segment of Social Structure 

Please note the following:

Sexual Selection

Sexual selection accounts for the development of characters that give individuals of the same sex gain an advantage over others of the same sex in competing for members of the opposite sex.  Darwin proposed that one dealt with "the power to conquer other males in battle" and another which is "the power to charm the females".  Today, these two modes are describe as:
  1. Intrasexual (macho man, e.g. John Wayne) which accounts for weapons and other fighting abilities that allow males to compete with one another in combat (literal or figurative) for mating rights.  It has never been seriously challenged as a way to account for the development of dimorphic characters that enhance fighting ability.
  2. Epigamic (pretty boy or Leonardo DiCaprio) or intersexual selection has to do with the development of characters (ornaments or behavior) that females prefer in mates and thus make them more attractive to females.  This mode is strongly based on active female choice.
Darwin never developed a good explanation of female choice in epigamic selection and why males would compete vigorously with one another for the right to mate with a female.  Trivers answered this question in a paper entitled “Parental Investment and Sexual Selection” where he predicted that "the sex that invests most in reproduction will be the choosier sex”.  Parental investment is defined as “any investment by the parent  in an individual offspring that increases the offspring’ s chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring”.  As a result, the following can be deduced: Trivers' Observation leads to some clear deduction about the nature of sexual behavioral differences between the sexes and the different criteria males and females use to assess members of the opposite sex as potential mates.

For example, since male investment limits female reproductive success, males should be deemed attractive if they are wealthy, powerful, or have high status.  On the other hand, males should be interested in females who have high reproductive value (emphasis on youth) and judged to be beautiful (an index of health and reproductive capability).  Buss in his cross-cultural study found the following:

Summary of Buss’ Research (Evolution of Desire)
 Buss’ cross-cultural survey found that in 36 of the 37 societies females more than males judged wealth as more important.

Predictions of Male and Female Differences in Sexuality (Don Symons The Evolution of Human Sexuality)

 

Inclusive Fitness (Kin Selection)

Kin selection theory or, more properly, inclusive fitness theory was devised by W.D. Hamilton in 1964 to explain apparent altruism in social species in Darwinian terms.  Darwin was troubled about apparent acts of self-sacrifice by individuals in highly social species.  He argued that if it could be shown for any species that traits had developed solely for the benefit of others in the population then his theory would be annihilated (note the term, not in need of modification or radical change but annihilate).  He felt this way because he argued that all individuals in a population were reproductively selfish and should not behave altruistically if in doing so they decreased their reproductive potential and enhanced that of another.  Altruistic behavior is then any behavior towards another which decreases one's fitness while simultaneously increasing the fitness of another.

Hamilton divided fitness into two components: (1) individual or Darwinian component - the sum of one's offspring (2) the kin component or the effect that ego's actions have on the reproductive success of relatives.  Together he argued they comprised one's inclusive fitness.  He then argued that an organism could behave altruistically (really selfishly) if the following inequality was satisfied and thus behave consistently with the expectations of natural selection theory:
(Bs - Cs) + r(Bk - Ck) > O.  (As modified by Irons in Chagnon and Irons, eds., 1979).
    Key "B" benefit, "C" cost, "s " to donor or altruist, "k" to recipient.

In practical terms this means that (all things being equal) one should be more willing to assist kin than non-kin or close kin compared to distant kin.  Critically, this does not mean that relatedness is the only consideration in the distribution of altruism.  It also depends on the costs of being altruistic (your loss of resources or increased exposure to risk) compared to the benefit to one's inclusive fitness.  For example, if you are wealthy, giving a grandchild $10,000 is not very costly to your fitness.  If you are deciding between two grandchildren (assume that the gift cannot be divided) then you must consider what impact the gift will have on their fitness since they are both equally related to you (i.e. by 1/4).  If one is nearly as rich as you and the other a struggling college student, it probably would be better to give it to the poorer grandchild since it will make a greater difference to the fitness of the latter compared to the former. However, if the latter is a drug addict who will squander the money on his vice, then it would probably be better to give it to the richer grandchild.  Even more to the point, it might be better to give it to a great grandchild (relatedness of 1/8) than to the wealthy grandchild.  The important thing to keep in mind is to understand the impact the gift or assistance will have on individuals as well as their relatedness.
 

Reciprocal Altruism

THE EVOLUTION OF RECIPROCAL ALTRUISM

While Hamilton has given us a theoretical account of how 'altruism' could evolve among individuals who are genetically related in various degrees (kin), Trivers has developed a model which attempts to account for the evolution of altruism among individuals who are genetically unrelated.
The chief way in which reciprocal altruism differs from kin selection (aside from relatedness [r] being set to zero) is the greater time lag in the return benefit to the giver of altruism who has suffered a cost to his fitness.  In the kin selection model the return of benefit for an altruistic act is argued to be immediate.  That is an individual giving is immediately repaid with an increment (often probabilistic) to his fitness (inclusive).  In reciprocal altruism, the case of A diving into the water to rescue a drowning  B, A (the altruist) cannot be immediately reciprocated by an increase to his fitness but must await an opportunity at some later point in time for B to return the favor.  The crucial problem for this model then is during this time lag will the recipient of aid reciprocate the aid.  He may decide not to reciprocate or to cheat (defined as breaking an agreement to reciprocate at some later point in time).

Trivers postulates that in order for reciprocal altruism to develop a variety of psychological mechanisms would evolve to
(1) detect cheaters;
(2) to enforce altruism in others;
(3) abilities to estimate the cost to self of any altruistic act and the benefit to self on the return for previous altruism.

 The following emotions are thought to have developed in response to the evolution of reciprocal altruism:
(1) the internalization of systems of moral rules which govern social behavior;
(2) the feeling of friendship and liking and disliking of individuals;
(3) moralistic aggression, or attempts at justification for failure to reciprocate because the cheater has not repaid his debt, etc.
(4) gratitude and sympathy degrees should depend on the cost and benefit of the altruistic act;
(5) guilt and reparative altruism should develop when one cheats, is discovered, and as a result one would feel the need to make amends;
(6) subtle cheating leading to attempts at manipulation of others (hypocrisy);
(7) detection of the cheater: the feelings of trust-worthiness and suspicion
(8) Developmental plasticity:  the ability to change one's behavior in respect to another depending on the history of reciprocal interactions.

The "Prisoner's Dilemma" problem presented in class is a way to conceptualize the problem of altruism and the tit-for-tat solution represents a robust strategy to guide reciprocal interactions.

 

 

Decay of the Nuclear Family

Chagnon investigated the stability of the nuclear family from the perspective of a child.  He found that by the time a child was age 15 he or she had less than a 50% chance of living with both biological parents.

The chart below shows the pattern for a number of family circumstance among the Yanomamö

The factors that lead to the "decay" of the nuclear family are composed of the following:

Since it is frequently argued that the "biological" nuclear family is the building block of all societies it would be useful to determine how common nuclear family decay is.  In this regard one would want to know whether the Yanomamö situation is common or rare cross culturally.  Unfortunately, the family of curves displayed by Chagnon for the Yanomamö have not been calculated for other populations.

Nevertheless, one can gain a estimation of nuclear family decay by comparing  Yanomamö divorce rates and mortality rates.

Mortality
    Although Yanomamö mortality rates are high compared to developed peoples they are not unusually high compared to traditional populations.

Divorce Rates
    We do not have much in the way of quantitative data on divorce rates in traditional societies.  Nevertheless, the data we do have indicates that Yanomamö divorce rates at 20% are in the bottom third of divorce rates for traditional societies and much lower than those found in modern societies.

Conclusion
    It is reasonable to conclude that the decay pattern found in Yanomamö families is not unusual.  This would suggest that the traditional nuclear family (note that it is defined in terms of biological parents and their children) is not as stable or fundamental as we have commonly supposed.
 

Resource Defense Polygyny

This model or explanation of polygyny suggests that female will be attracted to males who hold or control resources critical to her reproductive success.  It predicts that some males will become polygynous as a function of the resources they hold.  It also suggests that some polygynous female will have lower fitness than some monogamous females because it may be better for them to become the second wife of wealthy male than the sole wife of a poor males.

The graphical model as originally proposed by Orians-Verner-Willson is presented below:


 
 
 

The Problem of Polygyny

One widespread consequence, although not invariable, of polygyny is  reproductive depression of the women involved in such marriages.  This is especially common for the second wife.  This reproductive depression is only a problem from an evolutionary biological perspective.  Essentially one is forced to ask "Why would a woman choose a marital arrangement that would depress her fertility?"  This, of course, assumes that female choice (a concept from sexual selection theory) is operative.  This issue is taken up by Chisholm and Burbank in "Monogamy and Polygyny in Southeast Arnhem Land: Male Coercion and Female Choice"?  Ethology and Sociobiology 12:291-313 (1991).  They conclude that although male coercion of female marriage does effect women, they also note that female choice is also apparent through extra-marital liaisons which allow them to have selectivity in the men who sire their offspring.  Another strategy women employ may employ is sororal polygyny where sisters marrying the same man.  In these marriages, fertility is the same as in non-sororal unions but the survivorship of children who are siblings of co-wives is greater than siblings of unrelated co-wives.  Apparently, sororal co-wives cooperate in childcare activities more intensively and effectively than non-sororal co-wives.  At the proximate level, they note that lowered female fertility in polygynous marriages may be the consequence of marriage to older males (old males tend to be the polygynists) who have a lowered sex drive.

Beverly Strassman's recent report on the Dogon also documented the same pattern of depressed fertility among co-wives compared to monogamous women.  She demonstrated that the main factor was juvenile mortality rates of children in polygynous families.  Their mortality rates were 7 to 11 times greater than children in monogamous families.

She suggests that this may be true for four reasons: 

  1. Resource dilution.  Polygynous households may be wealthier overall but they are poorer on a per capita basis.  Data indicate that there are no per capita differences in wealth between polygynous and monogamous households.
  2. Cowife competition (the indigenous explanation): sons bring in wives and compete for lineage land while daughters leave the household upon marriage.  Cowives are not related and as a consequence poison the son’s of their cowife to insure that their son is able to get a larger fraction of lineage land when he marries.  No data on poisoning is presented although it is a common accusation and is sometimes brought to court and males have an extraordinary mortality rate compared to girls.
  3. Paternal investment: monogamous and polygynous males equally spend little time in direct care.  The main thing they can do to enhance the survival of children is to use money to pay for medicine to deal with chronic parasitic infections.  Although no data is presented, it may be the case that polygynous men are diverting money to gain additional wives for their own reproductive advantage.
  4. Nepotistic investment: it is possible that the number of kin differ for monogamous and polygynous children and that these kin would have an important effect on survival.  No data is presented.
She suspects that reason #3 (above) may be the main reason: polygynous husbands are less willing than monogamous husbands to invest their resources in medicines that would reduce the mortality rates of their children.  Instead, they may be more interested in using the money to establish a bride price fund to acquire yet another wife.
 

Gallup's General Findings on Patterns of Divorce in the Americas

 

Betzig's Cross-cultural Findings on Divorce

Basic point: reasons for divorce are commonly argued to be for the following reasons:
    Economics
    Reproduction
    Social
She argues that if one wants to understand the reasons for marriage, one may wish to  investigate the reasons for divorce since divorce would indicate that the most important reason for marriage is not being maintained.

Betzig concludes that reproductive issues are paramount reasons for divorce.  She therefore concludes that marriages are designed for reproduction.  Below is a rank-order list of reasons for divorce in an HRAF survey of 186 societies.

Causes of Conjugal Dissolution, a rank-ordering:
1. infidelity
2. infertility
3. personality
4. economics
5. in-laws
6. absence
7. health
8. ritual
9. politics

Some details of the findings under each reason for the top five

Infidelity
 Found in 88 (55%) of the 160 societies.

Divorce over adultery makes sense in economic, reproductive, and social theories of divorce.
Economic: infidelity associated with diversion of resources
Social: loss of status through clandestine and non-sanctioned sexual alliance
Reproductive: from the female point of view diversion of resources for child and spousal support and from the male point of view loss of paternity.  Furthermore, reproductive issues better explain the double standard.

Infertility
 Found in 75 (47%) of the 160 societies.
 So important is fertility that in some societies trial marriages occur until there is pregnancy or that marriage is not fully consummated until a child is born.
 If infertility occurs then there are a variety of alternatives which preclude divorce such as

Divorce for infertility follows directly from Darwinian considerations.
But social theories that children enhance prestige or economic theories which stress the value of children as economic assets are also supported by divorce caused by infertility.  So is polygyny as an alternative to divorce favor Darwinian, social, and economic theories.

Adoption and vicarious paternity are contra Darwinian theory and provide support for social and economic reasons for marriage.  But it should be pointed out that adoption tends to be of close kin (Silk) as are vicarious paters.

Personality
Displeasingness is reported as a cause in 51 (38%) of 160 societies.
Mutual consent in 42 (26%) as a cause in 160 societies.
“Displeasingness on the part of a husband or wife is very tantamount to no cause at all.” (p. 664).  Review of the ethnographic record indicates that these reasons have to do with infertility or infidelity.  Cases of sexual jealousy are quite common as when a man discovers that his wife is unfaithful he may act rotten to her.  Also, infertility leads to quarrels which lead to mutual consent.

Of course, it could be argued that personality conflict are the reasons for infidelity and infertility (lack of sexual contact).

Economics
Found in 29 (18%) of the 160 societies.
It has a sexually segregated nature in that for men it is the failure to provide basic resources while for women the failure to process resources.

In-Laws
Found in 25 (16%) societies of 160.
Include such things as inadequate service to in-laws (7), bridewealth and dowry disputes (6), disrespect of in-laws (3), and miscellaneous complaints (2).

Such causes are important for social theories of marriage but not inconsistent for economic theories of marriage or any of the others for they all emphasis the importance of kinship in economic or reproductive affairs.