Human Families Flashcards
What is parental investment
Any investment by a parent in offspring that increases the offspring’s likelihood of survival and reproduction at the cost of the parent’s ability to invest in/produce other offspring (Trivers 1972)
What are the different types of sex ratio
How can it be expressed
• BSR/ASR/OSR
=birth/adult (can be different from birth depending on mortality ratios) /operational
• Primary/secondary/tertiary
= fertilisation/BSR/ASR
Expressed as:
- ratio (number of males: 1 female)
- number of males per 100 females
- decimal (proportion of males)
What is the sex ratio in humans
Why may this be puzzling? Why what is the problem with this thinking?
The evolution of a 1:1 sex ratio,
approximately, usually
Wouldn’t it be more efficient to have a heavy female bias?
This is a group selection argument but we must remember evolution acts at the gene only
What is Fisher’s principle?
- Producing the rarer sex will be the best strategy for parents as the rarer sex has higher average reproductive success.
- Once the sex ratio is equal, the advantage disappears and the population is in equilibrium.
Automatically equilibrates
What are the assumptions of Fisher’s principle?
Assumptions:
- sexual reproduction
- random mating
- both sexes equally costly
Give 2 examples of animals which do not have an equal sex ratio
Woolly spider monkey - Sex ratio of 0.35
Western Tarsier - Ratio of 0.8?
Why may animals not have a balanced sex ratio
- Local Resource Competition (Clark 1978)
- Local Resource Enhancement (Komdeur et al. 1997)
Describe local resource competition
Given that individuals compete with others in the same group, if one sex is more likely to disperse, then this sex will be favoured
Females disperse so bias towards producing daughters in Woolly Spider Monkeys and vice versa for Western Tarsier
Describe local resource enhancement
When a certain sex increases resources in the environment so will be favoured
Some offspring birds remain in the nest to help further reproduction eg by guarding
How does the Seychelles warbler exemplify different reasons for an uneven sex ratio
- Local Resource Competition (Clark 1978)
- Local Resource Enhancement (Komdeur et al. 1997)
Depending on whether it was in a high quality resource location the driving force of the above 2 hypotheses change (in high quality places there is a bias towards daughters would remain local and help with cooperative breeding)
What is the Trivers & Willard Hypothesis
-male reproductive variance > female r.var
-high quality parents ought to produce more sons
(If you’re a winner its better to produce a son cos they will be a big winner than a daughter where there is less variance so it matters less )
• N.B. there are some species in which daughters
benefit more from maternal rank… eg in baboons
Why do baboons not fit the Trivers and Willard hypothesis
matrilineal inheritance
Mother ensures daughter is just below her in the hierarchy bc mother can outcompete anyone below her so can slot daughter in but son will disperse so are independent of mother rank
High ranking females will produce more daughters
Is there any evidence for adaptive sex determination in humans
Pastoralists with pronounced male skew
Society with very variable female health (1/4 the women have been estimated to have chronic energetic shortfalls)
Women with high BMI are much more likely to have a son (Mace and Gibson 2003) – fits with trivers wilard
Cameron and Dalerum (2009) – billionaires tend to have 60% sons, interestingly driven by male billionaires and not female billionaires – no clear explanation)
Mechanisms?
What are possible mechanisms for adaptive sex bias in humans
• Possibilities:
-male bias in production of x- vs y-bearing sperm.
-differential likelihood of successful fertilisation of x or y sperm
-differential likelihood of egg fertilised by x vs y sperm resulting
in completed pregnancy
• Numerous male and female hormonal influences of these processes see Navara 2013 e.g.
- F oestogen level > cervical mucosa viscosity > advantage for x sperm
- M testosterone:gonadotrophin > ratio of x:y sperm production
- Theories such as high ranking males have increased testosterone and increasing rank increases testosterone levels
• Glucose levels may inhibit female blastocyst development more so than males (Cameron 2008)…key driver of mechanism? - Increased energy intake prior to conception increases likelihood of male being born as glucose levels can inhibit female blastocyst development more than male
What is the tertiary sex ratio
Sex-ratio may also be altered after birth via parental behaviour e.g. infanticide/neglect/preferential treatment.
• Inuit female infanticide rates estimated at ~20% (Smith & Smith 1994). Male economic contribution and thus inclusive fitness contribution to parents much higher
“Life is short. We all want to be as prosperous as we can in the time we are alive. Therefore parents often consider that they cannot “afford” to waste several years nursing a girl. We get old so quickly, and so we must be quick and get a son. That is what we parents think, and in the same way we think for our children. If my daughter Quertilik had a girl child I would strangle it at once. If I did not, I think I would be a bad mother.” (Smith & Smith 1994)
Does the Triver Willard hypothesis hold up in India
what about china
Higher castes killed more girl babies (Dickermann 1979) bc triver hypothesis of high ranking males tend to produce sons bc they are bigger winners
But T-W predictions don’t consistently hold up:
-Lack of male infanticide in lower castes
-Among poor Chinese farmers majority of
infanticide directed at females,1y.o. SR reaching
375:100 (Hawkes 1981).
100 million missing women due to tertiary but now also due to sex specific abortions
No data on whether it is adaptive but unlikely bc there are surely many men who have no reproductive factors – short term proximal economic choice about labour etc
What are other causes of sex based infanticide
- dowry system
- sex differences in economic productivity
What did Dunbar find regarding biased sex investment in Hungary
• Empirical evidence that female biased PI is an adaptive strategy among Gypsies in Hungary due to hypergyny
(Bereczeki & Dunbar 1997).
• Daughters, particularly in urban population, produce more surviving grandchildren than sons because more likely to marry into wealthier native Hungarian population > lower child mortality risk.
• The gypsy population:
- F-biased sex-ratio at birth
- daughters breastfed for ~6 more months
- invest more in daughters’ education
Why is paternity uncertainty so important for male humans
• Paternity uncertainty has likely been an important selective force for men given the ubiquity of paternal investment in human populations.
Cuckoldry (where a male unwittingly invests in another male’s genetic child) is devastating for male fitness
Thus we would expect men to be endowed with some sort of psychological mechanism to protect against this by assessing paternity
Psychological experiments show that people tend to assure father’s far more than mothers that the baby looks like them
How common is cuckoldry
Non-paternity rates average ~3.3% - mixed sample, principally industrialised societies (Anderson 2006).
Paternity uncertainty may be higher in non-SIM social systems with partible paternity etc
Give an example of how putative fathers may use phenotypic matching to guide investment decisions and avoid cuckoldry.
- Photographs of same toddler morphed with five different adult faces including focal participant’s face.
- Self-resemblance had substantially more impact on male than female participants’ willingness to invest.
Platek et al. 2002
What did Alvergne 2009 find regarding paternity uncertainty
Took pictures of all men in the village and all children in the village then went to another village and showed pic of a child and 3 men, one of whom was the biological father. Did same with odours
Scored resemblance to the father based on how frequently correct child was chosen
Asked mothers about investment to create a paternal investment score
Investment score was positively correlated with odour and facial resemblance scores
Author tried to validate investment score by showing it was associated with child BMI and mid arm circumference
What is grandparental uncertainty predicted by (3)
- Number of uncertainty links (Differential
grandparental solicitude hypothesis; Euler &
Wizel 1996) - Number of uncertainty links compared to
other grandchildren (Preferential investment
hypothesis; Laham et al. 2005) - Marginal genetic relatedness due to shared
sex-chromosome (Sex chromosome selection
hypothesis; Chrastil et al. 2006)
Describe the uncertainty links in the differential grandparental solicitude hypothesis
Paternal grandfather uncertain by 2 degrees (not sure his son is bio son, and cant be sure grandchild is son’s bio child)
Paternal grandmother is 100% sure (no degrees of uncertainty because knows daughter is deffo hers and grandchild is deffo her daughter’s)
Therefore maternal grandmothers should invest most and paternal grandfather’s should invest least
Paternal grandmother may invest less than maternal grandmother bc there maybe other grandchildren who she can invest more in
Maternal grandfather more likely to invest than paternal grandmother (latter may have 100% sure grandchild, but former’s alternatives are even worse)
Think of alternative grandchildren
Which relative is most important for child survival
In a review of kin effects on child survival in 45 natural fertility populations, MGMs
found to be most important relative besides mother
Sear & Mace 2008
What did Marlowe find regarding how Hazda men interact with bio vs step children
Marlowe 1999
- Hadza men nurture, hold, communicate and play significantly more with biological children than stepchildren.
- Men living with stepchildren produce 877Kcal/day versus 1901Kcal/day of men living with only biological children.
How does being a step child affect your chances of survival
• Unequivocal evidence that stepchildren are at increased risk of physical abuse and neglect.
• National homicide archive of Canada, data from 1974-1990 indicate stepfathers more than 100X more likely than biological fathers to commit violent filicide. (Daly & Wilson 1996)
• Data from Australia indicate >300 fold increased risk (Strang 1996).
• Minimal confounding effects of other variables e.g. SES, family size, personality characteristics.
-When bio fathers kill resident children often associated with psych problems and followed by suicide whereas stepfathers kill without ether of these
Also see Marlowe’s 1999 finding that Hadza men play more and invest more calories in bio over step children
Summarise the lecture on biased parental investment
- Numerous adaptive reasons for biases in the BSR (LRC+LRE+TW). Some positive evidence for the TW hypothesis in humans.
- Female infanticide has produced substantially biased sex ratios and seems to be driven by economic interests of the family – labour contribution + dowry.
- Paternity certainty affects paternal and grandparental biases in investment based on cues of resemblance, number of uncertainty links and opportunity costs.
- Stepchildren at substantially higher risk of physical abuse and filicide.
What is Riet Ghok
(cattle talk) - Serious Business
When a youth wants to marry, the first question his sweetheart’s father asks him, or whoever is acting on his behalf, is: ‘What cattle have you got? ‘ although he generally knows the answer before asking it (Evans Pritchard 1946, p 256) [Nuer ethnography]
What is a marriage transaction
Economic exchange at point of marriage
Either between couple or their respective families
Marriage is usually a much longer, more complicated occurrence than in WEIRD societies
Describe the marriage transactions of the Nuer
Riet Ghok
Transfer of cattle from groom’s to bride’s family
Negotiate for many days/ weeks and many family members get in (pat. Uncles etc)
5-10 heads of cattle transferred -> initial official sign of path to marriage
More cattle brought at marriage and some cattle sacrificed bc both living and dead relatives are entitled of economic transactions
Marriage cannot be consummated until ~50% of total amount negotiated have been transferred
Even after this, bride will continue to stay with birth family until first child is born (sometimes around 50 cows can be transferred, around 1/3 of the male’s wealth has been transferred)
Once first child born then bride will move in
If childless there will be disputes about whether to dissolve to marriage and if cows should be returned etc
What are the different types of marriage transaction
How common is each
• Bridewealth – transfer of material resources from groom or his family to bride’s family.
-52% of SCCS/66% of Ethnographic Atlas
• Dowry – reversal of bridewealth
-6% of SCCS/3% of Ethnographic Atlas
• Brideservice – groom must provide labour/share products of labour with bride’s family for some
period of time.
-13% of SCCS
transaction is common in many small scale societies -
Most common in pastoralist societies
Usually some form of transaction
Sometimes equal transfer eg w1 bride from each family given to the other to cement alliance given the shared genetic interest (bidirectional)
Why is bridewealth expected to be common
what does it reflect
• Relative ubiquity of bridewealth expected since females are the limiting resource for male fitness not vice versa.
Bridewealth compensates family for loss of daughter and loss of additional worker
This occurs even where patrilocality isn’t the norm
• Among the Kipsigis higher brideprice is paid for females with higher reproductive value i.e. those reaching menarche earlier.
What is residual reproductive value
Residual reproductive value = number of future offspring female is expected to bear
How does brideservice work among the Kipsigis
Kipsigis pay bridewealth in cattle and cash and is based on reproductive value (translated to economic value)
Beginning of December, all girls who reach menarche undergo circumcision - then secluded so males can purchase one or multiple women as wives
Variable age of reaching menarche (12-20)
High payments biased towards those with earlier menarche (longer reproductive career) – difference of 8 years is significant = lots more children
What affects probability of dowry system emerging
Likelihood of dowry system emerging in stratified non-polygynous societies vs any other (including non stratified or stratified polygynous) is 50x more likely
PTM predicts differences in women’s access resources (held by men) largely cancelled out via polygyny.
Whether woman marries rich or poor will have a large impact on reproductive success
• If monogamy is imposed and there is substantial stratification in male wealth, women have to compete for wealthier husbands.
Gaulin and Boster, 1990
Which form of marriage transaction is most common in hunter gatherers
why
Brideservice
- Most HGs do not accumulate material resources – lack of storage systems + high mobility.
- Brideservice may be a parental test of potential son-in-law’s resource provisioning ability…higher variance in male productivity
- In some societies male prohibited from sexual relations until after brideservice is complete as offspring would restrict possibility of partnership termination. ‘Groom’ lives matrilocally - He is asked to hunt (eg asked to get honey in Nik’s HG tribe – honey is very risky
How do behaviours evolve
“If a pattern of behaviour causes people to produce more descendants who also follow that pattern, the behaviour will eventually come to predominate – or be naturally selected. This should hold regardless of the mechanism of transmission of the behaviour from parents to children.” (Hartung 1982)
Why brideservice and not groomservice?
Much more variance in male productivity than in females (women of same age more consistent)
Is this uniform? Hawks work on Hadza suggests it is maternal foraging ability that is a better predictor of offspring survival due to hunting being so luck based and requires so much skill that amount of resources will never form the basis of children’s diet
Byaka – parents disapproval tended to be disapproval of the daughter bc she didn’t work hard enough/ wasn’t a good enough gatherer
Prevalence of brideservice in HG societies where gathering is more important
How common are matrilineal societies
• 13/186 (17%) societies in SCCS are matrilineal.
How did social anthropologists attempt to explain the matrilineal puzzle
What was the bio anth problem with this
female descent + male authority > tension
between in-marrying male and his wife’s brothers over control of the
household and children
- matrilineal inheritance generally described as mother’s brother to sister’s son.
- why would men preferentially invest in sister’s children than own children given lower biological relatedness?
Could paternity uncertainty explain how matrilineal societies arise
At least man knows he is related to sister’s children than own children
Must be very low paternity certainty (p<0.268)
Would bring up lots of problems for marriage at any time
Paternity certainty would have to be unrealistically low to favour an avuncular over paternal effort
Relatively low paternity confidence is probably also a consequence as well as a cause of matriliny
What did Aberle say regarding matrilineal societies
“The cow is the enemy of matriliny and friend of patriliny” (Aberle 1961)
What was noticed about where matrilineal societies tended to emerge
- Only 8% of pastoralist/agro-pastoralist societies are matrilineal compared to 30% of horticultural societies.
- It was a question of kinship that inspired Galton’s problem.
- Societies are historically related > need to distinguish whether shared practices are a result of adaptation or shared ancestry.
How do we get around Galton’s problem in the study of culture
Cultural Phylogenetics
• To test functional hypotheses, we should
consider independent cases of evolution…not those resulting from shared ancestry.
• Language trees can be used to approximate shared ancestry.
• Test for correlated evolution along branches rather than correlation of traits.
When does matrilineality arise
Bs/Bd ratio – ability for sons to translate wealth into daughters: Holden et al. 2003 integrated TriversWillard logic and paternity certainty:
Bs/Bd < 1/P
Adaptive to invest in daughter rather than son when higher returns on male
investment do not outweigh risk of son being cuckolded.
Matrilineal systems evolve when parents gain more when they invest in daughters rather than sons
This is high in polygynous societies but lower in monogamous societies
When Bs/Bd is low then paternity certainty becomes increasingly important
Any level of paternity uncertainty favours investment in daughters
In horticulturalist societies land is not the limiting factor on production hence cannot be used to ‘buy wives’ in the way livestock can.
…Bs/Bd is much higher in pastoralist populations compared to horticultural ones.
this is Matriliny as Daughter Biased Investment
How is wealth passed on in matrilineal societies according to Matriliny as Daughter Biased Investment
hypothesis
• MDBI model suggests in matrilineal societies parents:
- transfer wealth to daughters [C]
- allow sons and daughters to use wealth but ensure it is inherited by daughters’ offspring [B]
• Inheritance from mother’s brother to sister’s son [A] is an illusion created by [B]…
the men never actually own the wealth, they just have temporary access whilst they are alive.
Original problem was mother’s brother would transfer resources to sister’s sons
Compare a patrilineal and matrilineal society
Gabbra camel herders in Kenya and the Chewa who are horticulturalists in Malawi (Holden)
Bs/Bd= 3 among Gabbra, largely because number of camels related to number of wives and therefore number of grandchildren
Bs/Bd=1, Chewa land is abundant and low productivity so harvest size isn’t determined by amount of land, but level of labour input into the land
Therefore, men can’t translate larger gardens into more wives
Wealth doesn’t lead to more grandchildren if invested in sons
Therefore, Bs/Bd doesn’t compensate for low paternity certainty (BS/BD, where BS is the benefit of a unit of wealth to ason, BD the benefit to a daughter)
critical value of P (probability of paternity) needed to make daughter biased wealth inheritance adaptive in that society (P< .36 in the Gabbra, P< .94 in the Chewa).
Thus you can see the Chewa are matrilineal and the Gabbra are patrilineal
Why may the matrilineal puzzle not be a puzzle
Mattison 2019 suggests the matrilineal puzzle rests on inaccurate assumptions:
- men do not actually have authority or ownership of resources in many cases (discrepancy between norms and reality)
- men tend not to invest much in next generation and when they do they do not favour nieces/nephews over own children.
- Under conditions where paternal investment is relatively unimportant (e.g. horticulture), men shift strategy towards mating effort and self-sufficient women are responsible for investment and resource transfers.
- Matriliny is association with male absence (e.g. due to warfare) and female contribution to subsistence.
Briefly describe what Mattison (2019) said of the matrilineal puzzle
Puzzle is illusory
Under certain circumstances where women are self-sufficient, pay off to men for paternal investment are low (bc women are able to do everything without them) so men focus on producing large quantity of children without investing in offspring or nieces/nephews etc
Therefore, matrilineal puzzle is just an artifact of the assumption that men control resources
Also fits with association with horticulture as this is the subsistence mode where women are responsible for a lot of the labour
Must remove this idea of male control and look at how important men and women are to subsistence
Summarise the lecture on marriage transactions
- Marriage transactions tend flow from the groom side to the bride side since females are the limiting resource on male fitness.
- Dowry occurs when intra-sexual competition operates more strongly on females due to cooccurrence of male wealth stratification & SIM.
- In foraging societies where there is minimal wealth accumulation, bridewealth is replaced by brideservice, which may also function as a son-in-law screening tool.
- Matrilineal avuncular inheritance is a long-standing puzzle with numerous explanations, including that the puzzle itself is illusory:
- paternity uncertainty
- different fitness returns on wealth for sons vs daughters (MDBI)
- female self-sufficiency
• Cultural phylogenetics provide a solution to Galton’s problem.
Summarise LHT
• Organisms harvest energy from the environment and convert it to replication efforts.
• Energy budget is finite thus involves trade-offs, the Darwinian demon doesn’t exist:
-Somatic (growth and maintenance) Vs Reproductive effort
-Quantity Vs Quality of offspring
• Optimal allocation varies across the life-course and environmental conditions; it shapes the schedule of development, reproduction and ageing.
Give 8 slow LHS traits
- Small adult size
- Fast Development
- Early reproduction
- Short gestation
- Many offspring/large litters
- Low parental investment
- Small offspring
- Short lifespan
Give 8 fast LHS traits
- Large adult size
- Slow Development
- Late reproduction
- Long gestation
- Few offspring/small litters
- High parental investment
- Large offspring
- Long lifespan
Give an example of the huge intraspecific variation in fertility rate in H. sapiens
- (married) Hutterite women average ~11 children; highest ever recorded (Eaton & Mayer 1953).
- In Soeul, S.Korea fertility rate is 0.83/woman.
Give an example of how energy allocation has knock on effects on many parts of LHS
Energy allocation has many knock-on effects meaning fast LHS leads to many of the same features eg small size where the organism will also die sooner because the body is less resilient/ robust, but also faster development because don’t have to grow as much (shorter gestation therefore can have large litter etc) – live fast, die young
Is the Quantity-quality trade off real
Lawson and Mace (2011)
most analyses show an increase in sibling number leads to a decrease in survival outcome
survival is just a means to reproduction in evolutionary terms
Therefore we must look at how number of siblings affects fertility
Becomes clear number of siblings doesn’t affect female fertility but does affect male fertility
Male fertility much more variable
Mixed effects
Men in HG societies with more siblings had HIGHER fertility (opposite of expected)
Men in pastoralist it is mixed:
More brothers = lower fertility
More sisters= higher fertility
Much more nuanced relationship when considering fertility
Trade off holds for survival but more nuanced for fertility
How does number of offspring affect fertility in different human societies
number of siblings doesn’t affect female fertility but does affect male fertility
Men in HG societies with more siblings had HIGHER fertility (opposite of expected)
Men in pastoralist it is mixed:
More brothers = lower fertility
More sisters= higher fertility
Much more nuanced relationship when considering fertility
Why is extrinsic risk important to the quality quantity trade off in LHT
Give 4 examples of extrinsic risk
Extent to which mortality is extrinsic/ uncontrollable (and so independent of parental investment) decreases the Q-Q trade-off
(Higher extrinsic risk means selection will act less strongly to restrict fertility - Fewer negative consequences for offspring if more are born )
- warfare
- natural disasters
- poor healthcare services + sanitation
- high pathogen stress
What did Desai find in relations to number of siblings and extrinsic risk
extrinsic risks can by extension reduce the impact of sibling competition. In her comparative study of developing populations,
using data from the Demographic and Health Surveys, negative effects of large sibship size on child height were weakest in populations with poor access to healthcare facilities or safe drinking water. In the absence of such initiatives, parents
may have limited ability to protect children from
environmental assaults, such as pathogen stress or crop failure, weakening any advantage to concentrating investment in relatively few offspring
Desai (1995)
How does juvenile help fit into LHT and family size theories
- Children may reduce the magnitude of sib competition if they can contribute to childcare/production.
- In no society have children been shown to completely offset their energetic cost.
- When there are safe and low-skill/strength production opportunities more scope for juvenile contribution and a reduction in Q-Q trade-off.
How may juvenile help differ between HG societies
varies based on ecology
arctic HGs where hunting takes lots of skill, children mostly reliant on parents for longer whereas in rainforests children collect high calorie fruit etc and contribute much more
the less children rely on parents, the less the effect of parents spreading investment over more children (lower QQ trade off)
How can intergenertional transfers affect sibling rivalry
• When parental contribution is a rival
good sib competition increases. E.g. family status/reputation is not subject to Q-Q trade-off.
• In wealth accumulating societies, rival material heritable resources intensify sib competition.
when material resources affect fitness, QQ trade-off is more pronounced
Who are the Gabra
How do they fit with evolutionary theories about family (3)
Mace (1996)
camel-herding, nomadic pastoralists who inhabit an arid area to the east of Lake Turkana, Kenya
Trivers Willard: no evidence of infanticide to bias investment (but this is an extreme prediction)
Wealth of a family (camel #): significant positive effect on the reproductive success of both men and women, although the effect of wealth is greater for men, as predicted from evolutionary theory
Effect of intergenerational transfers:
The greater the number of elder brothers a man has, the lower his RS, as a result of a smaller initial herd and a later age at marriage. This is not true for women - number of elder sisters does not have a measurable effect on a woman’s fertility, although it does have a small, negative effect on the size of her dowry
Why does having more elder brothers delay marriage and decrease household productivity and fertility among the Gabbra?
having lots of older brothers, who have already inherited much of the parental livestock, means younger brothers have fewer camels for their own family and must wait longer to accumulate enough camels for the bridewealth and to start their own family
How does sibling number affect male fertility in HGs and pastoralists?
Give an explanation
• Among Ache/!Kung HGs number of siblings increases male fertility, but among Gabbra and Kipsigis it decreases fertility.
• In
agriculturalist/ pastoralist populations material wealth is important for polygyny.
- Among foragers material wealth and polygyny is low/absent and relational wealth is important for dealing with food instability, childcare demands and political influence.
- Sibling help rather than competition in human evolutionary history?
Give an example of family optimisation
In the Dogón
- Likelihood of child mortality increases by 26% with each extra child in the nuclear unit.
- Max lifetime RS of 4.3 offspring obtained at fertility of 8.6 births.
- 75% of women achieved RS within CI of max.
- 83% of women had fertility level not significantly different from optimum; median fertility 8.5.
Strassman & Gillespie 2002
What does family size optimisation (Strassman & Gillespie 2002) show?
Since Dogon people are farmers, results do not support the assumptions that: (i) contemporary foragers behave more adaptively than agriculturalists, and (ii) that adaptive fertility behaviour ceased with the Neolithic revolution some 9000 years ago
What did Meji find regarding family size when studying a Ghana population
Meji (2009)
each additional child in the offspring group resulted in a 2.3% lower proportional survival of the offspring.
still found evidence for the trade-off when comparing pairs of co-wives, all living in polygynous compounds, it cannot account for the observed association (In the Dogon, Child mortality in polygynous households was higher than in monogamous households (Strassmann, 1997))
General pattern seems to be the more children a woman has the more surviving children there are
Even tho proportional survival is lower, fitness still maximised by maximising fertility
Suggests optimal behaviour would just be increasing fertility and this wouldn’t be limited in any way
How did having more inherited land affect Kipsigis women
women with more inherited land had higher esteem and better offspring survival (Borgerhoff Mulder, 2000)
Do humans optimize family size?
No?
- Even when Q-Q trade-off exists, majority of studies find RS is maximised by maximising fertility…actual fertility tends to be lower than optimal.
- The few studies that find an intermediate fertility level as optimal have failed to be replicated.
What are some methodological problems with studying human family size optimization
(we can’t do clutch experiments):
-fitness proxied by no. surviving offspring rather than no. grandchildren…survival ≠ quality.
- phenotypic correlation…mothers that can afford to have more offspring do masking sibling competition.
- reverse causality due to replacement/insurance effects.
What is a phenotypic correlation that should be accounted for when studying optimisation of human family size
Can’t assume women in a society have same resources etc – can’t assume same level of optimisation for every woman in a society
Eg women who have 14 children might just be those who can afford to have 14 while a poorer woman may limit births because increasing beyond her abilities may decrease number of surviving children or grandchildren
Ignoring intra-societal differences is massive oversimplification
What is the demographic transition
A substantial decline in the number of offspring produced…a shift from high fertility and mortality to low fertility and mortality that accompanies economic development.
How did Vining describe the demographic transition?
why? (2)
“the fundamental problem of sociobiology” (Vining 1986, p167):
- large decline in offspring produced despite a large increase in resource availability.
- the positive association between wealth and fertility within post DT societies disappears/reverses
What does Mace argue about optimisation in the demographic transition
Mace (1998)
Increasing the costs of raising children decreases optimal fertility and increases the inheritance left to each child at each level of wealth, and has the potential to reduce fertility to very low levels.
explains why wealthy families are frequently also those with the smallest number of children in heterogenous, post-transition societies.
Summarise Mace’s suggestions about fertility in post-DT societies (3)
Is this likely?
• In post-DT societies, costs of raising ‘competitive’ children are very high (Mace 1998;2014):
-education
-lack of labour contribution
• A country is not a single homogenous group, sociality and
marriage are structured by SES.
• Could low fertility could be adaptive when costs of raising
competitive offspring high?
• Poor empirical support that fertility reduction is actually
fitness maximising…need to shift focus to mechanisms
driving fertility
How has contraception been suggested to affect post-DT societies (2)
Is it likely? (2)
- Perhaps natural selection governed fertility via physiological mechanisms and sexual behaviour.
- Contraception has decoupled sex from reproduction resulting in maladaptive fertility declines.
- BUT European transition started well before contraception readily available.
- Many populations with contraception available have not undergone transition
In post-DT societies, Lack of positive association between wealth and fertility may be an artefact of looking at society as a homogenous group. What happens if we look at individual strata
Often socially imposed monogamy, fairly rare for individuals to be excluded from marriage market
Even if split into sub communities there is strong association between fertility and number of grandchildren
Reduced fertility unlikely to be maximising fitness
Probably a mismatch between modern conditions and psych/physiological mechanisms driving fertility in the past
Understanding proximate mechanisms that govern fertility behaviour and why this may be mismatched in conditions of post DT societies
Why may reduced fertility in post-DT societies be a mismatch
As pay offs in parental investment increase, number of offspring desired decreases (reduction in fertility)
When extrinsic risks increase, pay offs decrease
When risk is low, investment becomes more rewarding
Level of intrinsic risk drives fertility behaviour
External risk is now so low that response may be to have such a low number of offspring it becomes maladaptive
Ceiling for skill is very high, if parents have been selected to alter fertility based on how skilled their investment can make their offspring (usually adaptive as would be result in more children) but in market economy no longer translates to long term fitness
Kaplan et al. 1996
Why would fertility decline spread if it is maladaptive
cultural transmission (Boyd & Richerson 1985).
horizontal and oblique transmission + tends to be follow evolved shortcuts e.g. prestige bias/conformity.
• In market economies achieving high status may require sacrificing fertility; this low fertility behaviour may spread rapidly due to prestige bias (Colleran 2016)
Why may prestige bias have led to decreased fertility
High status no longer translates to more children
Rapid transmission of low fertility behaviour
Summarise lecture on family size and DT
• Natural selection shapes organisms to allocate energy across competing functions
(somatic vs reproductive; Q vs Q) to maximise fitness.
• Q-Q trade-off impacted by:
- extrinsic risk
- offspring contribution to subsistence
- importance of inheritance
- Evidence for optimization of family size in humans is weak but difficult to test.
- Demographic transition driven by mismatch between industrialised mkt economy environment and mechanisms driving fertility behaviour:
- payoffs to PI (low extrinsic risk; high ceiling of skill development)
- prestige bias (negative association between prestige and fertility)
What did Meyers claim about the DT
“The birthrate is a barometer of despair,” - loss of happiness in post-DT nations leading to fall in birthrate
How may Western culture act as a proximate mechanism for DT
Individualism is the important story/ cultural norm in many post-DT societies; this stems from religion (Buss, 2000)
adherence to in-group mentality important in CGS + higher quality offspring may increase group competition
religion important for success of these societies/ norms
easy to see how mismatch could arise
Discuss the results of the study on billionaires’ offspring
Cameron and Dalerum, 2009
male billionaires produce more children than female billionaires
An original fortune-maker had more grandchildren through his sons than through his daughters, supporting the assumption that reproductive success would be enhanced by giving birth to more sons than daughters.
women who were themselves billionaires, indicating high motivation, had fewer sons, and this was most marked among self-made women and those that were working and expanding their inheritance
Give a study about fertility talendencies in Kenya
Fertility is based on neighbourhood norms - conformist bias
Fertility fell from 6/woman to 4.8/ woman from 1948-2020 in Kenya -> rapid decrease
Iyer and Weeks 2020
Give studies regarding the importance of paternity certainty (4)
Holden et al. 2003 (integrated Trivers-Willard logic and paternity certainty: Bs/Bd < 1/P
Adaptive to invest in daughter rather than son when higher returns on male
investment do not outweigh risk of son being cuckolded.)
Anderson 2006 (Non-paternity rates average ~3.3% - mixed sample, principally industrialised societies ).
Platek et al. 2002 (• Self-resemblance had substantially more impact on male than female participants’ willingness to invest.)
Alvergne 2009 (Investment score was positively correlated with odour and facial resemblance scores)
grandparental uncertainty(1. Number of uncertainty links
(Differential grandparental solicitude hypothesis; Euler & Wizel 1996)
2. Number of uncertainty links compared to
other grandchildren (Preferential investment hypothesis; Laham et al. 2005)
3. Marginal genetic relatedness due to shared
sex-chromosome (Sex chromosome selection hypothesis; Chrastil et al. 2006))
What is cooperative breeding
does this fit with humans
cooperatively breeding species being those in which most of the females do not breed regularly and instead provide alloparental care to the offspring of a breeding female (typically only one) to whom they are genetically related, often as siblings or half-siblings (Lukas & Clutton-Brock, 2012 ).
Humans clearly do not meet this definition as virtually all women in traditional forager, horticultural and pastoral societies reproduce regularly if fecund and human breeding help is often de-coupled from relatedness
humans instead practice ‘‘biocultural reproduction’’ (Bogin et al., 2014)
Which cognitive ability do Bogin et al., 2014 stress is important in human biocultural reproduction
use of pretense to form fictive kinship eg give non-kin titles used to describe kin relationships
Chagnon (1992) reported that the Yanomamo¨ practice a type of kinship classification by which a man calls his
mother’s sisters and some other women by the kinship name ‘‘mother’’. The daughters of these ‘‘mothers’’ are not eligible for marriage
this allows lifetime reproductive effort to be reduced by up to 30%
Bogin 2014
