Sex differences in nepotism Trust in a trust game is associated with sex, exposure to baby’s and facial similarity Katinka Quintelier, Ghent University June 5, 2008 HBES 2008, Kyoto

Overview 1. 1.Introduction & Predictions 2. 2.Materials and Methods 3. 3.Results 4. 4.Discussion & Conclusions

Overview 1. 1.Introduction & Predictions 2. 2.Materials and Methods 3. 3.Results 4. 4.Discussion & Conclusions

1 Introduction  The fossil record of the genus Homo (Wood & Collard, 1999) versus earlier hominins suggests:  Slower maturation, prolonged development of children  Increase in brain size  Obligate bipedalism  Increase in body size Lake Turkana, Kenya Homo ergaster; Skull KNM-ER 3733 discovered by Bernard Ngeneo in 1975 (Kenya) Wood & Collard, The Human Genus. Science,284: Antón, 2003.Natural History of Homo erectus. Yearbook of Physical Anthropology, 46: Pictures:

1 Introduction  What also evolved since the emergence of the genus Homo:  Shorter lactation period & interbirth interval (Aiello & Key, 2002)  Very long dependency of children  Raising several dependent children simultaneously Lake Turkana, Kenya Homo ergaster; Skull KNM-ER 3733 discovered by Bernard Ngeneo in 1975 (Kenya)  This could not have evolved without a co-evolving change in social life, i.e. assistance by others than the mother in child care. (Hrdy, 2005) Aiello & Key, Energetic Consequences of Being a Homo erectus Female. American Journal of Human Biology, 14: Hrdy Evolutionary Context of Human Development. The Cooperative Breeding Model.

1. Introduction  Who would assist a mother to decrease the burden of child care?  C < rB (Hamilton, 1964)  Father  paternal care  Kin  cooperative breeding  “Cooperative breeding is a breeding system in which group members, other than the genetic parents (alloparents), help one or both parents rear their offspring”.  Hrdy, Hamilton, The genetic evolution of social behavior. J Theoretical Biology 7: pp Geary, Evolution and Proximate Expression of Human Paternal Investment. Psychological Bulletin 126, 1: Hrdy, 2005.

Sear and Mace, Who keeps children alive? A review of the effects of kin on child survival. Evolution and Human Behavior 29:1-18. E.g. Kramer, Children’s Help and the Pace of Reproduction: Cooperative Breeding in Humans. Evolutionary Anthropology 14: Introduction   Mother   Maternal grandmothers   Paternal grandmothers   Older siblings Sear and Mace, 2008.

1. Introduction  Women are expected to be more nepotistic than men. Reproductive succes can be increased by :  Increasing survival of offspring (getting help from kin)  Increasing inclusive fitness (helping kin)  Increasing quantity of offspring  Some data support this hypothesis.  (LA) Women give more help to and receive more help from wealthy female kin with children. (Essock-Vitale & McGuire, 1985)  (Canadian) Sisters recalled more relatives and refered more to kinship status in characterizing themselves, than their brothers did. (Salmon & Daly, 1996) Neyer & Lang, Blood is Thicker Than Water: Kinship Orientation Across Adulthood. J PersSoc Psychol 84, 2: Salmon & Daly, On the importance of Kin Relations to Canadian Women and Men. Eth & Soc 17: Essock-Vitale & McGuire, Women’s Lives Viewed from an Evolutionary Perspective. II. Patterns of Helping. Eth &Soc 6:

1. Predictions   If we elicit a context of cooperative breeding, women will be more cooperative, at least when they are possibly related.   Female students will be more trusting towards another female,   When they are first exposed to pictures of baby’s   And when the other female looks subtly similar to them.

1. Predictions   Is facial resemblance a possible kinship cue?   Facial resemblance enhances trust (e.g. DeBruine, 2002).   Facial resemblance enhances cooperation (Krupp et al., 2008).   Facial resemblance of other-sex faces increases trust but decreases their attractiveness in the context of a short-term relationship (DeBruine, 2005).   An implicit evaluation of relatedness. DeBruine, Facial resemblance enhances trust. Proc. R. Soc. Lond. B. 269: DeBruine, 2005b Trustworthy but not lustworthy: context-specific effects of facial resemblance. Proc. R. Soc. B. 272: Krupp, DeBruine & Barclay, A cue of kinship promotes cooperation for the public good. Journal of Evolution and Human Behavior:49-55.

Overview 1. 1.Introduction & Predictions 2. 2.Materials and Methods 1. 1.Design 2. 2.Participants 3. 3.Stimuli 4. 4.Procedure 3. 3.Results 4. 4.Discussion & Conclusions

2.1. Design  Subject’s sex  2 conditions  Picture evaluation task  2 conditions: landscape or baby  Trust game  2 conditions: resemblance or no resemblance  « Player 2 » was always female!  2 x 2 x 2, all between subjects variables  The entire program ran in «Presentation 12.1.»

2.1. Design  Subject’s sex  2 conditions  Picture evaluation task  2 conditions: landscape or baby  Trust game  2 conditions: resemblance or no resemblance  « Player 2 » was always female!  2 x 2 x 2, all between subjects variables  The entire program ran in «Presentation 12.1 »

2.1. Design  Subject’s sex  2 conditions  Picture evaluation task  2 conditions: landscape or baby  Trust game  2 conditions: resemblance or no resemblance  « Player 2 » was always female!  2 x 2 x 2, all between subjects variables  The entire program ran in «Presentation 12.1 »

2.1. Participants   45 male, 47 female undergraduate students   Mean age = 21.54; s.d. = 2.97   European

2.2. Stimuli   Trust game   Two conditions   Condition 1: player 2 resembles participant   Condition 2: player 2 does not resemble participant

Player 2 Base face Minear & Park, A lifespan database of adult facial stimuli. Behavior Research Methods, Instruments, & Computers, 36 (4): Stimuli Facial Stimuli Database

Player 2 Base face 2.2. Stimuli Image manipulation software: Psychomorph (Tiddeman et al., 2005) Transformation method cf. DeBruine, % Shape only transformation Player 2 Tiddeman, Stirrat & Perrett, Towards realism in facial transformation: results of a wavelet MRF method. Computer Graphics Forum, Eurographics conference issue, Vol 24, No 1-5. DeBruine, Facial resemblance increases the attractiveness of same-sex face more than other-sex faces. Proc. R. Soc. Lond. B, 271: Minear & Park, Subject

DeBruine, Tiddeman, Stirrat & Perrett, Minear & Park,  50% 50% Shape only transformation Player 2 Base face Shape of subject’s face Shape of same-sex composite face 2.2. Stimuli Player 2  50%

Player 2 Base face DeBruine, Tiddeman, Stirratt & Perret, Minear & Park, Stimuli  50% Shape of subject’s face Shape of same-sex composite face 50% Shape only transformation

2.3. Procedure   Trust game subject Player 2

X 3! subject Player 2 subject Player Procedure

Player 2 Subject 2.3. Procedure

Overview 1. 1.Introduction & Predictions 2. 2.Materials and Methods 3. 3.Results 4. 4.Discussion & Conclusions

3. Results

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Overview 1. 1.Introduction & Predictions 2. 2.Materials and Methods 3. 3.Results 4. 4.Discussion & Conclusions 1. 1.Female Subjects 2. 2.Male Subjects

4. Discussion & Conclusions   The data seem to support the prediction that extended child dependency shaped cooperative behaviour of related women.   They are consistent with other data (Essock-Vitale & McGuire, 1985).   Proximate mechanisms? Essock-Vitale & McGuire, 1985.

4. Discussion & Conclusions Key & Aiello, A Prisoner’s Dilemma Model of the Evolution of Paternal Care. Folia Primatologia, 71:   The data seem to be consistent with other data (Key & Aiello, 2000), suggesting that cooperative behavior of men towards women can evolve     When women face a relatively high cost of reproduction.   When there is a link between cooperation and reproduction, e.g. mating effort.   Other explanations?   Proximate mechanisms?

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