bunny rabbit puppet
© elenacastaldi77 / Getty Images
If you are breaking a promise to a child, be sure to have a solid reason.

Pre-school children can tell the difference between a reasonable explanation and cop out when it comes to breaking promises.

In a paper published in Cognitive Development, psychologists from Duke University in the US studied the responses of 64 children, aged three to five, after animal puppets promised to show them a cool toy - and then failed to do so.

Afterwards, the puppets either gave a good excuse for going back on their word ("I had to help my friend with his homework"), a bad excuse ("I wanted to watch TV"), or no explanation at all.

The children were then asked whether they thought the puppets' actions were wrong or not, and why.

In all cases, children agreed it was wrong to break a promise.

Yet they were more understanding when the puppet offered a good (prosocial) excuse versus a lame, or selfish one. It shows children realise social obligations can sometimes take priority.

"Kids are on to you. They know when you're giving a bad excuse," says author Leon Li.

But the quality of a puppet's excuse didn't affect whether the children liked a puppet or not.

The results show even very young children are capable moral reasoning, such as distinguishing between good and bad reasons for breaking a promise.

The study is part of a larger field of research on how children come to appreciate and act on cultural and moral norms for how we behave and treat each other.

"Morality is a type of common ground that we have with others, with mutual expectations about how we should behave, and what counts as good grounds for justification," Li says.

"We're showing that young children become attuned to this common ground at an early age."

Li says the findings are also relevant to any adult who has uttered the classic fallback phrase, "Because I said so."

"Kids are paying attention and can tell that is a lame reason."