Friend or Foe? Early Social Evaluation of Human Interactions

<div><p>We report evidence that 29-month-old toddlers and 10-month-old preverbal infants discriminate between two agents: a pro-social agent, who performs a positive (comforting) action on a human patient and a negative (harmful) action on an inanimate object, and an anti-social agent, who does the converse. The evidence shows that they prefer the former to the latter even though the agents perform the same bodily movements. Given that humans can cause physical harm to their conspecifics, we discuss this finding in light of the likely adaptive value of the ability to detect harmful human agents.</p></div

Average indexes for 29 months-olds.

<p><b>A.</b> Average Absolute Valence Index for 29-months-old toddlers computed over the responses to the individual questions regarding the pro-social agent and the anti-social agent separately. *p<.05 (between subjects),**p<.02 (within subjects). A positive score indicates a positive verbal statement toward the agent and vice versa for negative scores. The error bars correspond to one between-subject standard error above and below the mean. <b>B.</b> Average Relative Valence Index computed over the responses to the contrastive questions. A positive score indicates a more positive assessment of pro-social than the anti-social agent, vice-versa for negative scores. The error bars correspond to one between-subject standard error above and below the mean. *p<.05.</p

Set up and average preference index for 10-month-olds.

<p>A Setup of the experiment for the 10-month-olds. Infants are seated in their parent’s lap (who are blindfolded) during the experiment. After the presentation of the sequences described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088612#pone-0088612-g001" target="_blank">Figure 1</a>, infants see a novel sequence, where the two actors appear on stage, stare at the camera, play with two identical toys, and present them to the infants. Simultaneously, two real toys appear on the table through a mechanical device. The infants’ first attempt to reach one of the toys is coded by two independent scorers. Infants are tested 4 times with different toys, and the actors switching places on the screen. B. Average preference index for 10-month-olds computed over the 4 response trials. A positive score indicates a preference towards the toy of the pro-social agent, a negative score, a preference towards the anti-social agent. The error bars correspond to one between-subject standard error above and below the mean. *p<.05.</p