Sex differences in Sex differences in nepotismnepotism

Trust in a trust game is associated with sex, Trust in a trust game is associated with sex, exposure to baby’s and facial similarityexposure to baby’s and facial similarity

Katinka Quintelier, Ghent UniversityKatinka.Quintelier@UGent.be

www.themoralbrain.be

June 5, 2008HBES 2008, Kyoto

Overview

1. Introduction & Predictions

2.Materials and Methods

3.Results

4.Discussion & Conclusions

Overview

1. Introduction & Predictions

2.Materials and Methods

3.Results

4.Discussion & Conclusions

1 Introduction

The fossil record of the genus The fossil record of the genus HomoHomo (Wood & (Wood & Collard, 1999) versus earlier hominins suggests: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, 1999. The Human Genus. Science ,284:65-71.•Antón, 2003.Natural History of Homo erectus. Yearbook of Physical Anthropology, 46:126-170. •Pictures: http://commons.wikimedia.org/wiki

1 Introduction

What also evolved since the emergence of the genus What also evolved since the emergence of the genus HomoHomo::

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, 2002. Energetic Consequences of Being a Homo erectus Female. American Journal of Human Biology, 14:551-565. •Hrdy 2005. Evolutionary Context of Human Development. The Cooperative Breeding Model.

1. Introduction

Who would assist a mother to decrease the burden of child care? Who would assist a mother to decrease the burden of child care?

C < rB (Hamilton, 1964)C < rB (Hamilton, 1964)

Father Father paternal care paternal care

Kin Kin cooperative breeding cooperative breeding

““Cooperative breeding is a breeding system in which group Cooperative breeding is a breeding system in which group members, other than the genetic parents (alloparents), help one members, other than the genetic parents (alloparents), help one or both parents rear their offspring”.or both parents rear their offspring”.

Hrdy, 2005.Hrdy, 2005.

•Hamilton, 1964. The genetic evolution of social behavior. J Theoretical Biology 7: pp. 17-18•Geary, 2000. Evolution and Proximate Expression of Human Paternal Investment. Psychological Bulletin 126, 1:55-77.•Hrdy, 2005.

•Sear and Mace, 2008. Who keeps children alive? A review of the effects of kin on child survival. Evolution and Human Behavior 29:1-18.•E.g. Kramer, 2005. Children’s Help and the Pace of Reproduction: Cooperative Breeding in Humans. Evolutionary Anthropology 14:224-237.

1. Introduction

Mother Maternal

grandmothers Paternal

grandmothers Older siblings

Sear and Mace, 2008.

1. Introduction

Women are expected to be more nepotistic than men. Reproductive Women are expected to be more nepotistic than men. Reproductive succes can be increased by :succes can be increased by :

Increasing survival of offspring (getting help from kin)Increasing survival of offspring (getting help from kin) Increasing inclusive fitness (helping kin)Increasing inclusive fitness (helping kin) Increasing quantity of offspringIncreasing quantity of offspring

Some data support this hypothesis.Some data support this hypothesis.

(LA) Women give more help to and receive more help from wealthy female kin with (LA) Women give more help to and receive more help from wealthy female kin with children. (Essock-Vitale & McGuire, 1985)children. (Essock-Vitale & McGuire, 1985)

(Canadian) Sisters recalled more relatives and refered more to kinship status in (Canadian) Sisters recalled more relatives and refered more to kinship status in characterizing themselves, than their brothers did. (Salmon & Daly, 1996)characterizing themselves, than their brothers did. (Salmon & Daly, 1996)

Neyer & Lang, 2003. Blood is Thicker Than Water: Kinship Orientation Across Adulthood. J PersSoc Psychol 84, 2:310-321.Salmon & Daly, 1996. On the importance of Kin Relations to Canadian Women and Men. Eth & Soc 17:289-297.Essock-Vitale & McGuire, 1985. Women’s Lives Viewed from an Evolutionary Perspective. II. Patterns of Helping. Eth &Soc 6:155-173.

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, 2002. Facial resemblance enhances trust. Proc. R. Soc. Lond. B. 269: 1307-1312•DeBruine, 2005b Trustworthy but not lustworthy: context-specific effects of facial resemblance. Proc. R. Soc. B. 272:919-922.•Krupp, DeBruine & Barclay, 2008. A cue of kinship promotes cooperation for the public good. Journal of Evolution and Human Behavior:49-55.

Overview

1. Introduction & Predictions

2. Materials and Methods

1. Design2. Participants3. Stimuli4. Procedure

3. Results

4. Discussion & Conclusions

2.1. Design2.1. Design

Subject’s sexSubject’s sex 2 conditions2 conditions

Picture evaluation taskPicture evaluation task 2 conditions: landscape or baby2 conditions: landscape or baby

Trust gameTrust game 2 conditions: resemblance or no resemblance2 conditions: resemblance or no resemblance « Player 2 » was always female!« Player 2 » was always female!

2 x 2 x 2, all between subjects variables2 x 2 x 2, all between subjects variables

The entire program ran in «Presentation 12.1.»The entire program ran in «Presentation 12.1.»

2.1. Design2.1. Design

Subject’s sexSubject’s sex 2 conditions2 conditions

Picture evaluation taskPicture evaluation task 2 conditions: landscape or baby2 conditions: landscape or baby

Trust gameTrust game 2 conditions: resemblance or no resemblance2 conditions: resemblance or no resemblance « Player 2 » was always female!« Player 2 » was always female!

2 x 2 x 2, all between subjects variables2 x 2 x 2, all between subjects variables

The entire program ran in «Presentation 12.1 »The entire program ran in «Presentation 12.1 »

2.1. Design2.1. Design

Subject’s sexSubject’s sex 2 conditions2 conditions

Picture evaluation taskPicture evaluation task 2 conditions: landscape or baby2 conditions: landscape or baby

Trust gameTrust game 2 conditions: resemblance or no resemblance2 conditions: resemblance or no resemblance « Player 2 » was always female!« Player 2 » was always female!

2 x 2 x 2, all between subjects variables2 x 2 x 2, all between subjects variables

The entire program ran in «Presentation 12.1 »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, 2004. A lifespan database of adult facial stimuli. Behavior Research Methods, Instruments, & Computers, 36 (4):630-633.

2.2. Stimuli

Facial Stimuli Database

http://agingmind.utdallas.edu/

Player 2 Base face

2.2. Stimuli

•Image manipulation software: Psychomorph (Tiddeman et al., 2005)

•Transformation method cf. DeBruine, 2004.

50% Shape only transformation

Player 2

• Tiddeman, Stirrat & Perrett, 2005. Towards realism in facial transformation: results of a wavelet MRF method. Computer Graphics Forum, Eurographics conference issue, Vol 24, No 1-5.• DeBruine, 2004. Facial resemblance increases the attractiveness of same-sex face more than other-sex faces. Proc. R. Soc. Lond. B, 271:2085-2090. • Minear & Park, 2004.

Subject

•DeBruine, 2004. •Tiddeman, Stirrat & Perrett, 2005.•Minear & Park, 2004.

50%

50% Shape only transformation

Player 2 Base face

Shape of subject’s face

Shape ofsame-sex composite face

2.2. Stimuli

Player 2

50%

Player 2 Base face

• DeBruine, 2004.•Tiddeman , Stirratt & Perret, 2005.•Minear & Park, 2004.

2.3. Stimuli

50%

Shape of subject’s face

Shape ofsame-sex composite face

50% Shape only transformation

2.3. Procedure

Trust game

subject Player 2

X 3!subject Player 2

subject Player 2

2.3. Procedure

Player 2

Player 2

Subject

Subject

2.3. Procedure

Overview

1. Introduction & Predictions

2.Materials and Methods

3.Results

4.Discussion & Conclusions

3. Results

Source Type III Sum of Squares df Mean Square F Sig.

Corrected Model 40.282(a) 7 5.755 2.386 .029 Intercept 1315.307 1 1315.307 545.355 .000 sex 5.079 1 5.079 2.106 .151 contextual_cue 12.061 1 12.061 5.001 .028 similarity 7.111 1 7.111 2.948 .090 sex * contextual_cue 2.095 1 2.095 .868 .354 sex * similarity 1.954 1 1.954 .810 .371 contextual_cue * similarity .821 1 .821 .340 .561 sex * contextual_cue * similarity 10.621 1 10.621 4.404 .039

Error 195.359 81 2.412 Total 1573.000 89 Corrected Total 235.640 88

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Overview

1. Introduction & Predictions

2. Materials and Methods

3. Results

4. Discussion & Conclusions

1. Female Subjects2. 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, 2000. A Prisoner’s Dilemma Model of the Evolution of Paternal Care. Folia Primatologia, 71:77-92.

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?

Thank you!Thank you!