Infants understand the idea of a fair share - and who does the work, deserves the reward - from as young as 17 months old.
That's according to University of Auckland researchers who demonstrated the response in children of that age for the first time ever.
In a recently published study, Dr Annette Henderson and Dr Ying Wang observed 84 infants using a dedicated video lab where the children were shown a sequence of different short scenes played out by two puppets.
In all of the scenes, the puppets worked together to access an out-of-reach bowl of sweets using plastic stacking cups.
After they got the bowl, the puppets divided the sweets up between themselves.
Scenes shown demonstrated principles of "distributive justice" - where the puppet putting in most of the work took the most sweets - while other scenes showed disproportionate rewards, where one puppet did more work but took fewer sweets.
A hidden video camera recorded each child's response to the scenes played onscreen, measuring "looking time" – the length of attention paid to each scene.
Responses were recorded and analysed using a dedicated software programme.
The researchers expected that infants would spend more time looking at scenes that violated justice principles compared to those that did not.
They found infants looked longer at scenes where the puppet who contributed more work took fewer sweets but, critically, not when the puppet who did most of the work took the biggest share.
"These findings suggest that infants didn't perceive events as a simple matter of negativity bias," Henderson said.
That is, simple inequality in distribution did not attract their attention significantly but where rewards were unequally or unfairly distributed, the children paid more attention.
"The research was the first time infants as young as 17 months had been tested on concepts such as equity and equality where the "actors" could themselves decide how resources should be shared.
Previous studies found 21-month-olds were sensitive to principles of distributive justice but in those studies resources were distributed by a third party.
"These findings are significant as they demonstrate that a fairly nuanced understanding of fairness is present much earlier in development than previous research has shown," Henderson said.
"By 18 months, infants expect individuals to take what that they deserve."
In everyday life, Wang added, humans expected resources will be distributed equitably and this sense of fair play had evolved as a function of humans' co-operative propensity.
"These findings demonstrate that infants are ready to learn about fairness and sharing behaviours within the first year and a half of their lives," Wang said.
"Finding ways to promote these behaviours early in life could be the key to enhancing fairness and prosocial behaviour in human social groups."
The research has been published in the Journal of Experimental Child Psychology.