Do toddlers prefer that agents help similar or dissimilar needy agents?

Recent research revealed rich socio-cognitive abilities in toddlerhood. The current study investigates whether 21-month-olds expect that wealthy agents help agents needing food or shelter. By using the violation of expecta- tion (VoE) implicit paradigm, toddlers' expectations on help- ing behaviors were explored upon the arrival of agents that need food (food condition) and shelter (shelter condition) in a within-subjects design. Two experiments were carried out, wherein, in one, the wealthy and needy agents were morphologically similar (Experiment 1), in the other one, the wealthy agents were morphologically different from needy agents (Experiment 2). The results showed that when needy agents are morphologically similar to wealthy agents, tod- dlers looked longer at the event that displayed the wealthy agents help needy agents only in food condition. When needy agents were morphologically different from wealthy agents, toddlers looked longer at the event that showed wealthy agents help needy agents in both food and shelter conditions. These findings demonstrate an interplay between similarity and helping behaviors in association with biological needs, and shed light on possible evolutionary mechanisms underlying prosociality.