Spontaneous prosocial choice by chimpanzees
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Spontaneous prosocial choice by chimpanzees. Horner et al. Background - Altruism. Altruism commonly seen in humans Chimps closest biological relative Contradiction between observational and experimental studies Altruistic or empathetic behaviors seen observationally (sharing food)

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Spontaneous prosocial choice by chimpanzees

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Spontaneous prosocial choice by chimpanzees

Spontaneous prosocial choice by chimpanzees

Horner et al.

Background altruism

Background - Altruism

  • Altruism commonly seen in humans

  • Chimps closest biological relative

  • Contradiction between observational and experimental studies

    • Altruistic or empathetic behaviors seen observationally (sharing food)

    • Lack of consistent evidence for prosocial behavior in controlled experimental conditions

  • Two primary altruistic experimental methods

    • Giving Assistance Tests (GAT)

    • Prosocial Choice Tests (PCT)

Background giving assistance tests

Background - Giving Assistance Tests

  • Choice between giving help or doing nothing

    • Giving an out-of-reach object for a human or conspecific

    • Provided a conspecific with a needed tool

  • Bonobos in particular show prosocial tendencies in GAT

  • Communication greatly increases chance of receiving needed help

    • Children similarly generally grant help only to vocal partners

Background prosocial choice tests

Background - Prosocial Choice Tests

  • Subject chooses between two outcomes that are identical to self

    • One choice rewards only self, the other rewards self and a peer

  • Chimps have not shown consistent preference for the prosocial choice

  • However other monkeys (e.g. capuchin monkeys) have shown prosocial preference

Background current study

Background – Current Study

  • Confounding factors may be responsible for lack of significant results

    • Complex testing apparatus

    • Distracting rewards

    • Limited contact between subject and partners

  • Horner et al.’s goal: reduce these extra factors

Methods overview

Methods - Overview

  • Subject chooses a token from the bucket

  • Tokens represent either selfish or prosocial choice

  • Food given to either just subject, or both subject and partner, based on token

Methods participants

Methods - Participants

  • 7 Adult female chimpanzees (6 in data)

  • All members of a long-established 12 chimp living group

  • All experienced in a token exchange program

  • Each chimp was tested with three different partners

    • One with a significantly affiliative relationship

    • One with a significantly negative relationship

    • One with a neutral relationship

  • Each subject given a preference tests, to control for any color preference in the tokens

Methods contingency training

Methods – Contingency Training

  • Two participants were chosen and randomly designated as either the actor or the partner

  • Actor given a set of 10 tokens, 5 selfish and 5 prosocial (distinguished by differing color)

  • Experimenter requested them back one-by-one

  • Actor rewarded when giving any token back, partner rewarded when prosocial token given back

Methods pct

Methods - PCT

  • Subject given tub filled with 30 tokens, 15 of each

  • Hands a token to experimenter, who puts token on easily visible platform, and replaces token in tub

  • Experimenter gives banana treat, wrapped in a noisy paper wrapping, to either the subject or the subject and partner

  • Partner can be easily seen and heard

Methods control trials

Methods – Control Trials

  • Subjects were run through experiment again, with identical procedures until the PCT, including preference test and contingency training

    • A new color scheme selected for tokens

  • PCT the same, except for the lack of a partner

    • If a prosocial token chosen, experimenter still pretends to give a food reward to an imaginary partner



  • Prosocialv.s. selfish token

    • Bias towards prosocial choice

  • No partner control group

    • chimps choose randomly





  • Found no correlation for choices based off of previous behavior when roles were reversed

  • High ranking chimps found to be more prosocial than subordinate apes

  • No significant difference between related individuals vs. non related

  • No correlation between prosocial behavior and outside affiliation (grooming pairs)

Results actor partner interactions

Results- Actor/partner interactions

  • Chimps interacted frequently

  • Behavior of partner categorized as:

    • Neutral

    • Attention-getting

    • Direct requests and pressure (DRP)

    • Partners engaged in the non-neutral behaviors more often following selfish choices by the actor

Results actor partner interactions1

Results- Actor/partner interactions

Results actor partner interactions2

Results- Actor/partner interactions

  • Actors acted prosocial towards neutral partners, and even more prosocial towards attention-getting partners

  • Actors did not act differently from chance level for DRP partners

Results actor partner interactions3

Results- Actor/partner interactions



  • Chimps favor the prosocial option

  • Factors such as kinship, affiliation, dominance rank or reciprocity have no effects

  • Matches what chimps do in the wild when sharing a common goal



  • However, this study does have different results than other studies done in a lab

  • They believe this was due to

    • Close proximity

    • Wrapping the treats in paper

    • Avoiding complex apparatuses

    • Results also differ from popular suggestion that prosocial choices happen more often between related monkeys



  • They do acknowledge that chimps could be acting on reciprocity for events happening outside of the experiment

  • How do you think this could be controlled for?

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