Presented by: Kyle Dennison and Victor Nguyen

Introduction: Kin Influence Hypothesis  Kin Influence Hypothesis:  Communication between people who are related is more likely to contain encouragement for individuals to behave in ways consistent with achieving reproductive success.  Communication with people who are not related is less likely to contain such encouragement.  Therefore interacting with relatives increases the probability that individuals will behave in more effective pursuit of reproductive success.  This study tested this assumption of the hypothesis.

Models of Cultural Evolution  Innate biases that influence the transmission of information can alter cultural norms.  Culture evolves in a way that can be predicted by the bias, even if the bias is weak.  Because kin do not play as important a role in modern societies, this may account for the demographic transition.  The authors present a model showing that reducing interaction with kin can lead to a loss of “pronatal cultural norms.”  Previous studies have focused on “learning” biases, the Kin Influence Hypothesis assumes “teaching” bias.

Modern Social Networks and Fertility  Modern transportation allows people to travel far from home and interact more with non-relatives.  Wider social networks are related to the adoption of behavior that is not best for attaining reproductive success.  It’s possible limiting family size is adaptive for other reasons, but there is not sufficient evidence to conclude that greater investment in fewer children in modern societies equals greater reproductive success.  Another explanation is that evolved psychological mechanisms are to blame for the decline of fertility.

The Cooperative Nature of Human Reproduction  Problem: Why wouldn’t humans have evolved mechanisms to limit the influence of others (culture) if not doing so could limit fitness?  Possible answer: The cooperative nature of human reproduction. Humans are only reproductively active a small portion of life. Those not in a reproductive phase help females who are raise children. A social network is a resource for successfully raising young. Because a woman’s reproductive decisions affect the group, decisions harmful to the group may be “punished” Establishing consensus on appropriate reproductive behavior would have lead to greater reproductive success.

The Cooperative Nature of Human Reproduction Continued  A social group consisting of many individuals with a stake in encouraging reproductive success the consensus will most like be that which maximizes inclusive fitness.  In traditional societies, norms are what individuals would choose if they were trying to maximize their own inclusive fitness.  Groups that are made up of people with no interest in each other’s fitness (non-relatives), will reach a consensus that reflects other shared or individual goals.

Methods  It is not possible (or ethical) to examine peoples’ private social interactions that would reflect the norms of interest for this study.  In answer to this problem the researchers decided to use a role- play technique.  The goal was to see if women in the “mother” role were more likely to push for choices in line with reproductive success.  Participants read one of 16 scenarios in which a childless woman in her reproductive years asked an older woman if she should have a child in her situation.  Participants wrote: 1) What they thought the older woman would say to the younger woman 2) What they themselves thought the younger woman should do.

Methods Part 2  In 8 of the scenarios the pair of women are a mother and a daughter.  In the other half they are just close friends a generation apart in age.  Only the young woman’s situation varies while the older woman’s stays the same.  In half of the scenarios the young woman is in her early 20’s and in the other half she is in her early 30’s.

Younger Female’s Circumstances  Her circumstances were portrayed in four different ways: 1) Career Couple: The woman and her husband both have successful careers (and fun) and she is not sure if she wants a baby. 2) Stepmother: She is the stepmother of 2 kids and she possibly wants to have her own child with her husband. 3) Widow: The woman married her boyfriend after finding out he was dying. She wants a child to keep part of him with her. 4) Single Woman: She has a successful career but no partner and is considering having a child to raise on her own.

Design  There were 4 dichotomous I.V.’s that could influence participant opinion on whether the girl should get pregnant: 1) Situation (Easy or Difficult): The Career Couple and Stepmother stories represent easy conditions, the Widow and Single Woman scenarios depict harder situations. 2) Norms (Normal or Deviant): Single Woman depicts a deviant norm while the Widow scenario represents a “normal” norm. 3) Relationship (Mother or Friend): Each story was told in two versions, with the older woman being a mother or a friend. 4) Age (Younger or Older): Each of the 4 stories had two versions, younger 20’s (old enough to get pregnant later) and younger 30’s (nearing the end of reproductive cycle).

Design Part 2  What participants wrote provided 2 ordinal dependent variables. 1) Answer: Did their response include the answer “yes” (1), “no” (- 1) or no definite answer (0) about whether the woman should become pregnant? 2) Advice: Was the advice likely to sway the woman toward trying to become pregnant (1), away from trying to get pregnant (-1), or no advice/neutral advice (0). Values were assigned by two women, one in her 50’s and another in her 20’s. There was a 3 rd DV; what the participants themselves thought the woman should do.

Participants  379 women with age ranging from 25 to greater than 75  Recruited from University of Exeter staff and alumni  About two thirds of the participants were mothers.  Almost 80% were British and the rest were from other developed nations.  Participants had to be fluent in English and able to use a computer.  Potentials were directed to a website that explained the study  Those who agreed were randomly linked to 1 of 16 sites that presented them with a scenario and collected responses.  Between 20 and 28 responses were collected for each scenario.

Predictions  The Kin Influence Hypothesis Predicts that: 1) Women in the mother role will be more likely than those in the friend role to be positive about pregnancy when the situation is “easy” and less likely when the situation is “difficult.” 2) When pregnancy will break a social norm both mother and friend are predicted to be less positive about pregnancy. 3) Age is also predicted to have an effect:  “Mothers” will be more likely to encourage pregnancy in difficult or deviant situations if the young woman is in her 30’s (near the end of her reproductive years).

Results  Difficult (widow + single) > Easy (career + stepmother) 12x in reply 25x in own opinion  Nicola in twenties = 8x in own opinion  Nicola in thirties = 4x in reply

Results  Participant is not a mother > participant is a mother in own opinion  In difficult situation (widow + single) As friend > As mother ○ 3.56:1  In easy situation (career + stepmother) As mother > As friend ○ 2:1

Results  Age (Nicola is in her twenties vs thirties) In difficult situations, ○ Age not very important In easy situations, ○ 30 y.o. > 20 y.o.  Deviance (single mother vs widow) Single mother < Widow ○.13:1

Discussion  Purpose: See if situation has an effect on reproduction evaluations  Support of kin influence hypothesis Mother role more likely to believe Nicola should get pregnant in easy versus difficult situations. Situation less important as friend. Single mother influence on participants’ beliefs

Discussion  Unfound support for kin influence hypothesis (but not refuted either) Age of Nicola did not influence those playing the role of mother. (Belief regarding children near end of reproductive cycle) ○ Women able to have children later (biological reality versus cultural expectation) ○ Lack of life experience in 20s > fear of childlessness in 30s

Modeling  Two traits to consider: h versus l  B K favors h, B N favors l  Kin interaction promotes h, inverse true  Kin weight is A, non-kin weight is (1-A)  Kin interactions increases high fertility  Non-kin interactions decrease high fertility  Equilibrium point possible