Conflict between parents and children & 'wicked stepmothers': Parental Investment Author Paul Kenyon Reflective exercise Hamilton's kin selection theory Paternal investment Parent-child conflict Conflict between offspring The 'Wicked stepmother effect' Parent-offspring conflict before birth Seminar topic: Unwanted children Abortion Infanticide Stepparents Locating mothers of abandoned babies Caring for abandoned babies Does equality of parental investment disadvantage middle-borns? Seminar topic: Unwanted parents Adolescence and the souring of parent-child relationships The Oedipus Complex Seminar topic: Investment from relatives Reflective exercise review Relative's investment in children Resolution of parent-offspring conflict: An evolutionary unstable "arms race" References Online resources Further reading

Reflective exercise

This lecture considers the relationships between children and members of their family. Before you begin, you may find it useful to focus on who you are related to. This is not an experiment. There are no right / wrong answers. Your responses will not be recorded.

Write the first names of your relatives (if applicable) in these boxes :

Now think about how much these people are concerned about your welfare.

We will return to consider the pattern of your results later.

Hamilton's kin selection theory

Before beginning this section, view the video from Scientific American Frontiers "Masked Killers. Stronger Masked Booby chicks kill their smaller siblings to better ensure their own survival, and that of their mothers as well."

Relatives (kin) share many common genes. Genes can spread by benefiting other carriers of the same gene. Hamilton proposed the inclusive fitness or kin selection theory to explain altruism or self-sacrifice.

In an altruistic encounter there is:

• an altruist or donor who bears a cost ( c )
• a recipient who gets a benefit ( b )
• a degree of genetic relationship between the donor and recipient

The probability that the altruist and the recipient share a gene is called the coefficient of relatedness ( r ). The diagram shows the extent to which we share genes with our relatives. The value of r varies between 0 and 1. On average we share half of our genes with our brothers, sisters and children ( r=0.5 ), and a quarter of our genes are identical with those of our grandchildren, nephews and nieces ( r=0.25 )

According to Hamilton's Rule altruism pays off if rb>c . In other words, shared genes will profit if the cost to the altruist is less than the benefit to the recipient multiplied by the probability that the recipient shares genes with the donor.

Costs and benefits are expressed in units of fitness or reproductive success with values between 0 and 1.

• A cost of 1 unit of fitness means that the act would reduce the donor's reproductive success by 1 offspring.
• A benefit of 1 unit of fitness means that the recipient would increase their reproductive success by 1 offspring.

For the sake of argument assume you have spare food that you could give to your brother to feed him and his children.

• Assume that the cost to you is 0.1 units of fitness (i.e. if you do this 10 times you will have one less child)
• Assume that the benefit to your brother is 0.25 (i.e. if he receives 4 such donations he will have one more child)

We can test if your altruism would benefit kin selection by putting these values into Hamilton's Rule rb>c where:

• r ( the coefficient of relatedness between you and your brother) = 0.5
• c (the impact on your reproductive success ) = 0.1
• b ( the benefit to your brother's reproductive success) = 0.25
• rb =0.5x0.25=0.125
• c=0.1
• because rb>c (0.125 is greater than 0.1) Hamilton's Rule is satisfied and your altruism would benefit your genes

You might wonder why b and c are not always equal. Why not use the spare food you have to increase your own reproductive success? Well there is a limit to how much you can eat. If you have an abundance of food and your brother is starving, the cost to you of sharing is small, but it may be a matter of life or death to your brother and his children.

Paternal investment

According to a simple version of kin selection theory, men should give more resources to their genetic children than to stepchildren. Anderson et al (1999) looked at the odds that a man would give financial support to  'children' who were attending college.   A man's children were classified into one of four types

Compared to a child fathered by the man, and whose mother was the man's previous partner (Class 2 child), a man is 2.27 times more likely to support a child through college if he was the father of the child and his current partner is the child's mother.  a man is very unlikely to support a child through college if the child's mother was a previous partner, and the man was the child's stepfather Both of these effects are statistically significant. One surprising finding is that a man does not discriminate financially between a child who was born to the man's current partner in a previous relationship (Class 3 child), and a child fathered by the man in a previous relationship (Class 2 child) even though the man is genetically related to the class 2 child, but is only the stepfather of the Class 3 child Men may invest in the children of their current partners in order to convince the current partners that they are 'good providers' and thus persuade their current partner to bear him further children. Men may invest less in their own children from previous relationships because they are not sure that they are the true fathers of the children born whilst they were living with their  previous partners: 'paternity uncertainty'

Parent - child conflict

Trivers applied Hamilton's mathematical formula for kin selection to within-family conflict.  According to Trivers: "Parents are classically assumed to allocate investment in their young in such a way as to maximize the number surviving, while offspring are assumed to be passive vessels into which parents pour the appropriate care. Once one imagines offspring as actors in this interaction, then conflict must be assumed to lie at the heart of sexual reproduction itself-an offspring attempting from the very beginning to maximize its reproductive success would presumably want more investment than its parent is selected to give" (Trivers, 1974).

Here is a section from Buss (1999) which illustrates how conflict can arise from the application of the principles of kin selection.

"Suppose you have one sibling who has the same reproductive value as you. Your mother comes home from a day of gathering with two food items to feed her children. As with many resources, there are diminishing returns associated with each increase in consumption - that is, the value of the first unit of food consumed is higher than the value of the second unit of food. The first unit of food, for example, may prevent starvation, whereas the second unit of food just makes you a little fuller and fatter. Let's say that the first item would raise your reproductive success by four units and the second item of food would raise it an additional three units. Your sibling's consumption of these food items would have the same result, with diminishing returns associated with each added food item.

Now comes the conflict. From your mother's perspective, the ideal allocation would be to give one unit of food to you and one to your sibling. This would net her eight units of increase, four for you and four for your sibling. If either you or your sibling monopolized all the food, however, the gain would only be seven (four for the first item plus three for the second). So from your mother's perspective an equal allocation between her children would yield the best outcome.

From your perspective, however, you are twice as valuable as your sibling--you have 100 percent of your genes, whereas your sibling only has 50 percent of your genes (on average). Therefore, your mother's ideal allocation would benefit you by the four units that you receive plus only two of the units that your sibling receives (since you benefit by only 50 percent of whatever your sibling receives), for a total of six units benefit. If you manage to get all the food, however, you benefit by seven units (four for the first item plus three for the second). Therefore, from your perspective the ideal allocation in this simplified example would be for you to get all the food and your sibling none. This conflicts with your mother's ideal allocation, which is to distribute equally, however. The general conclusion is this: The theory of parent-offspring conflict predicts that each child will generally desire a larger portion of the parents' resources than the parents want to give. Although the above example is simplified in various ways, this general conclusion applies even when siblings differ in their value to the parents and even when the parents have only a single child. If the parents were to go along with the ideal allocation of resources desired by the child, it would take away from other channels through which the parents might be reproductively successful. Interestingly, parent-child conflict over the parent's resources is predicted not merely to occur at particular times such as adolescence, but at each stage of life (Daly & Wilson, 1988).

The theory of parent-offspring conflict yields a number of specific hypotheses that can be tested: (1) parents and children will get into conflict about the time at which the child should be weaned, with the parents generally wanting to wean the child sooner and the child wanting to continue to receive resources longer; (2) parents will encourage children to value their siblings more than children are naturally inclined to value them; and (3) parents will tend to punish conflict between siblings and reward cooperation. " Buss (1999).

This table shows how various allocations of food affect each relative's inclusive fitness

Assume that:

• the mother has two offspring:
  • offspring A
  • offspring B
• the coefficient of relatedness between the mother and each of her offspring is 0.5
• the coefficient of relatedness between offspring A and offspring B is 0.5
• mother has 2 indivisible units of food to give to her offspring
• eating 1 unit of food would increase an offspring's fitness by 4 units
• eating 2 units of food would increase an offspring's fitness by 7 units i.e. eating more food has diminishing returns

Conflict between offspring

Before beginning this section, view the video from Scientific American Frontiers "Masked Killers. Stronger Masked Booby chicks kill their smaller siblings to better ensure their own survival, and that of their mothers as well."

Note that although this scenario is often described in terms of parent-offspring conflict, there is also conflict between the offspring for the mother's resources.

Potential conflict between siblings is magnified if they are only half siblings i.e. the offspring have separate fathers, but the same mother. This table shows how reducing the coefficient of relatedness from 0.5 (full siblings ) to 0.25 (half siblings ) affects the fitness outcomes.

Assume that:

• the mother has two offspring:
  • offspring A
  • offspring B
• Offspring A and offspring B share the same mother, but have different fathers
• the coefficient of relatedness between the mother and each of her offspring is 0.5
• the coefficient of relatedness between offspring A and offspring B is 0.25
• mother has 2 indivisible units of food to give to her offspring
• eating 1 unit of food would increase an offspring's fitness by 4 units
• eating 2 units of food would increase an offspring's fitness by 7 units i.e. eating more food has diminishing returns

The 'Wicked stepmother effect'

Parent-offspring conflict begins before birth

Parent-off spring conflict begins at the moment a mother's egg is fertilized by the father's sperm (see Buss, 1999). Approximately 78% of all fertilized eggs fail to implant, or spontaneously abort early in pregnancy - often before the mother is aware that she is pregnant. High blood pressure in pregnancy (which may lead to the potentially fatal condition preeclampsia) is due to the foetus secreting the hormone hGC (human chorionic gonadotrophin) into the mother's bloodstream. This increases the mother's blood pressure, which in turn delivers more nutrient-containing blood to the foetus. Thus this mechanism benefits the foetus at the expense of the mother. Maternal blood glucose level can rise after a meal in late pregnancy despite the fact that maternal insulin level also increases. It has been suggested that the placenta produces hormones that decrease the mother's sensitivity to insulin, and consequently blood glucose increases to benefit growth of the foetus (see Cartwright 2000, p 266 ). The foetus is a battleground for its parent's genes (see Rossano, 2003). Imprinted genes are unusual genes that only express their traits if they are inherited from one parent or the other. The gene for Igf2 (insulin-like growth factor 2) is only effective if it is inherited from the father. This gene increases growth of the foetus. The foetus contains 50% of its father's genes. Expression of Igf2 will benefit the father's genes at the expense of the mother who has to provide nutrition for the developing offspring. Remember that the mother's genes (50%) are just as likely to be passed on if she is bearing a lighter foetus Mothers can contribute a gene called M6p which tends to act against the resource demands placed on the mother by the father's Igf2 gene.

Seminar topics
Unwanted children

Thinking about unwanted children releases a range of strong emotions in all of us. When dealing with this distressing topic it is useful to bear in mind the caveat that just because evolutionary psychology reveals something about human behaviour, this does not mean that human behaviour 'ought' to be this way, or that the behaviour is somehow biologically determined and inevitable.

Read Hrdy (2001). The Past,Present,and Future of the Human Family. The Tanner Lectures on Human Values Delivered at University of Utah February 27 and 28,2001. Available online  and consider the following questions: Distinguish between  'essentialist' and 'social constructionist' views of maternal behaviour. Is maternal devotion to children socially constructed? What is 'wet-nursing'? Give an example to support the claim the "the same genotype (can) be very variably expressed depending on local ecological and historical conditions." Hrdy (2001, p 65). Explain, with  examples, the terms 'species-typical' and 'sex-typical' human universals. Give estimated survival rates for babies born in 18th century Paris who were: wet-nursed at home wet-nursed outside the home deposited in foundling homes. Is the onset of maternal behaviour after childbirth inevitable and immediate ? Describe the roles of smell, prolactin and oxytocin in human maternal bonding. Is a breast-feeding mother more, or less, likely to abandon her baby? Explain - with examples - the terms: allomother, alloparent, cooperative breeding. Are humans cooperative breeders? Do men make good parents? Are unwanted children at greater risk of developing deviant behaviours?

Abortion

Read Barrett et al (2002) Chapter 7 Parental Investment Strategies and consider the following questions: What environmental factors are likely to contribute to a person's decision to foster, abandon or kill unwanted offspring? What are the implications of your answer for public policy on the care of unwanted children? Why do you think young, single women are more likely to abort a pregnancy than married women of the same age? What are the implications of your answer for social policy? Read Rossano (2003) Females and the Mixed Strategy. In Chapter 12 (Cooperation Between the Sexes III: The Female Perspective ),Evolutionary Psychology: The Science of Human Behavior and Evolution, and consider the following questions: Why do teenage mothers come disproportionately from economically deprived urban areas? What contribution can evolutionary psychology make to sex education in schools?

Figure redrawn from Barrett et al (2003)

Infanticide

Infanticide is a relatively rare phenomenon, but it has been documented in virtually every human society (see Barrett et al 2002). Infanticide is the solution of last resort for people who need to drastically reduce parental investment. Read Barrett et al (2002) Chapter 7 Parental Investment Strategies and explain, with example, how each of the following situations could lead to infanticide: lack of paternal certainty poor offspring quality lack of parental resources Read Cartwright 2000, section 10.3.2 Infanticide) and consider the following questions: Why does the risk of infanticide at the hands of the mother decrease as a function of the child's age? What other explanations could account for this effect? Why is the risk of infanticide greater for boys than girls? Why is a child at greater risk of infanticide from its mother than father? Why is a child at greater risk of infanticide from its parents, than other blood relatives or strangers?

Figure redrawn from Cartwright (2000)

Step-parents

"Having a step-parent has turned out to be the most powerful epidemiological risk factor for severe child maltreatment yet discovered." (Daly and Wilson, 1988 cited in Cartwright 2000 p 276).

Read Cartwright 2000, section 10.3.2 Infanticide) and consider the following questions:

• Can these findings be explained in terms of paternal investment by step-parents?
• Evaluate other explanations for these findings.

Locating mothers of abandoned children

In mid-December 2002 the body of a baby, inside a black plastic bag, was found by contractors clearing a site of rubbish outside a village in Cornwall. The post-mortem revealed that the baby girl lived for a brief time, but died from neglect. The police must contact the mother because they are concerned for her welfare. They have launched house-to-house enquiries in the area. Police and Health Service representatives made a television appeal to the mother, and for help from the general public. If you were approached by the police to suggest some characteristics of the mother in order to help them narrow-down their search for the mother, what advice you give? You should justify each characteristic in your profile by reference to the research literature.

Caring for abandoned babies

In Germany desperate mothers who do not want their babies are able to drop them off anonymously through a chute at a Hamburg day care centre. A silent alarm sounds, a camera watching the carrier turns on, and the baby is monitored until a nurse arrives. The program, “Operation Foundling,” was launched in Hamburg with the intent of reducing the growing number of babies left abandoned.

• A British psychologist - interviewed as part of the news item on the baby abandoned in Cornwall - suggested that there was no need for a similar scheme in the UK.
• "History warns us that if we set up depositories for "unwanted " infants human mothers are likely to avail themselves of them." Hrdy (2001)

  Comment on these suggestions with reference to the literature you have read.

Does equality of parental investment disadvantage middle-borns?

Hertwig, Davis and Sulloway (2002) provide a model which shows that even when parents allocate investment equally between their children, it can still lead to inequalities as a consequence of a child's birth order.

The following table introduces three imaginary couples who all started their families in 1970. We will use these families to see how birth order affects allocation of resources within a family

'Mr and Mrs Singleton' had one child born in 1970
'Mr and Mrs Doubleday' had two children. Their first child was born in 1970 and they had another baby five years later in 1975
'Mr and Mrs Trimble' had three children, the first born in 1970, a second was born in 1975, and their last child arrived in 1980.

All three sets of parents were determined to allocate an equal amount of their resources (love, care, attention, time and money) to each of their children, and they supported each child until it was 19 years old.

For the sake of illustration, assume that each family has 100 units of resource to allocate to their children during any five year period.

This table shows how the impact of allocating resources equally between children in each of our families.

Years........	1st born	1st born		1st born
1970-1974	100	100	2nd born	100	2nd born
1975-1979	100	50	50	50	50	3rd born
1980-1984	100	50	50	33	33	33
1985-1989	100	50	50	33	33	33
1990-1994			100		50	50
1995-1999						100
Cumulative investment over 19 years	400	250	250	217	167	217
Investment in first five years of life	100	100	50	100	50	33
Investment in last five year of rearing	100	50	100	33	50	100

Note that equal investment in each child:

Read Hertwig, Davis and Sulloway (2002). Parental Investment: How an Equity Motive Can Produce Inequality. Psychological Bulletin, 128/5, 728-745. Available online and consider the following questions: Make a list of examples of resources provided by parents to children under five, and over fifteen years of age. What are the underlying assumptions in Hertwig et al's equity model? How do the duration of parental care interbirth interval and family size affect resource handicap? Is there evidence for first born middle born and last born resource handicap?

Seminar topic: Unwanted parents

Adolescence and the souring of parent-child relationships

There is a paradox at the heart of the relationship between parent and child. Adult fertility declines with age, therefore with the passage of time and accumulated parental investment, children become more and more valuable to their parents because they are able to reproduce. But at the same time, parents become a burden and impediment to increasingly independent children.

Read Developmental Patterns in Parent-Child Relationships in Rossano (2003) Chapter 14 and consider the following questions: What is 'daughter guarding'? Why do some fathers encourage their sons to 'sow wild oats'? Why do you think most teenagers have difficulties in their relationship with their parents? Why do you think these difficulties are usually eventually resolved?

The Oedipus Complex

Read "The Oedipal Complex Revisited" and 'Killing Parents and the Asymmetry of Valuing Parents and Children' in Buss (1999, Chapter 7)

The brutal murder of Jeremy Bamber's family shocked the nation in August 1985. With Bamber standing to inherit his adoptive family's £500,000 fortune, it was inevitable suspicion would fall on him. But Bamber, now 41, has always denied shooting his parents, his sister and her six-year-old twin sons at their Georgian farmhouse in Tolleshunt D'Arcy, Essex. Bamber said his sister, who had not been taking her medication for mild schizophrenia, had "gone crazy", shooting her parents and children before killing herself. Sentencing Bamber to five life prison terms, the judge Mr Justice Drake said he was "warped and evil beyond belief". Bamber's appeals against conviction were rejected in 1989 and 2002 (From BBC News report: Thursday, 12 December, 2002) After studying evolutionary psychology, and hearing the keywords: adoption, inheritance, sister, schizophrenia, suicide, and nephews, would you consider that you could act as a unprejudiced juror in this case? State the reasons for you conclusion. Is Jeremy Bamber evil?

Investment from relatives

Reflective exercise review

Review the information you provided about the relationship you provided about the relationship between you and your relatives at the beginning of this topic, bearing in mind the following questions: Are / were you closer to your grandmother on your mother, or your father's, side of the family? Are / were you closer to your grandfather on your mother, or your father's, side of the family? Are / were you closer to your aunt(s) on your mother, or your father's, side of the family? Are / were you closer to your uncle(s) on your mother, or your father's, side of the family? If you have children of your own, you may wish to consider how you perceive their relationships with their grandparents, uncles and aunts.

Relative's investment in children

Read Buss (1999 p236-) and consider the following questions:

• List the various ways in which relatives can invest in each other's welfare.
• Is your relatives 'gekummert' towards you consistent with that seen in other studies?
• Can you see a pattern in your relatives concern for your welfare?
• Can the pattern of results be explained in terms of a sex difference in caring for children e.g.:
  • grandmothers are more interested in the welfare of children than grandfathers?
  • aunts are more interested in the welfare of nieces and nephews than uncles ?
• What is the function of the human family?

Resolution of parent-offspring conflict: An evolutionary unstable "arms race"

Read Kolliker and Richner (2001) Parent-offspring conflict and the genetics of offspring solicitation and parental response. Animal Behaviour, 62, 395-407. Available online.

References

• Anderson, Kaplan and Lancaster (1999) Paternal Care by Genetic Fathers and Stepfathers I: Reports from Albuquerque Men. Evolution and Human Behavior 20: 405–431. Available online
• Barrett et al (2002). Human Evolutionary Psychology, Palgrave, Basingstoke.
• Buss (1999). Evolutionary Psychology: The New Science of the Mind, Allyn & Bacon, Boston.
  • Buss (1999). The Theory of Parent-Offspring Conflict. In Evolutionary Psychology: The New Science of the Mind, Allyn & Bacon, Boston. Section available online
• Cartwright, (2000). Evolution and Human Behaviour, Macmillan Press, Basingstoke, 2000
• Gaulin & McBurney, (2001). Psychology: An Evolutionary Approach , Prentice Hall, New Jersey, 2001
• Hertwig, Davis and Sulloway (2002). Parental Investment: How an Equity Motive Can Produce Inequality. Psychological Bulletin, 128/5, 728-745. Available online
• Haig (1993) Genetic conflicts in human pregnancy. The Quarterly Review of Biology, 68, 495-532.
• Hrdy (2001). The Past,Present,and Future of the Human Family. The Tanner Lectures on Human Values Delivered at University of Utah February 27 and 28,2001. Available online
• Rossano (2003). Evolutionary Psychology: The Science of Human Behavior and Evolution. Wiley.
• Trivers (1971). The evolution of reciprocal altruism. Quarterly Review of Biology , 46, 35-57, 1971. Available online.
• Trivers (1974). 1974. Parent-offspring conflict. American Zoologist 14: 249-264
  

Online resources

• Anderson, Kaplan and Lancaster (1999) Paternal Care by Genetic Fathers and Stepfathers I: Reports from Albuquerque Men. Evolution and Human Behavior 20: 405–431. Available online
• Buss (1999). The Theory of Parent-Offspring Conflict. In Evolutionary Psychology: The New Science of the Mind, Allyn & Bacon, Boston. Section available online
• Daly and Wilson (1996). Evolutionary Psychology and Marital Conflict: The Relevance of Stepchildren. In D.M. Buss & N. Malamuth (Eds.), Sex, power, conflict: feminist and evolutionary perspectives. New York: Oxford University Press, pp 9-28. Available online
• Daly and  Wilson. (1998). The Truth About Cinderella. New Haven: Yale University Press. Excerpts available online
• Hrdy (2001). The Past, Present, and Future of the Human Family. The Tanner Lectures on Human Values. Delivered at University of Utah, February 27 and 28,2001. Available online.
• Hertwig, Davis and Sulloway (2002). Parental Investment: How an Equity Motive Can Produce Inequality. Psychological Bulletin, 128/5, 728-745. Available online
• Kolliker and Richner (2001) Parent–offspring conflict and the genetics of offspring solicitation and parental response. Animal Behaviour , 2001, 62, 395–407. Available online
• Scientific American Frontiers (1999). "Masked Killers. Stronger Masked Booby chicks kill their smaller siblings to better ensure their own survival, and that of their mothers as well." video available online
• Trivers (1971). The evolution of reciprocal altruism. Quarterly Review of Biology , 46, 35-57, 1971. Available online.