Someone assembles a Lego tower for a child and then topples it. Before the child learns to say thank you to the person who assembled that tower, they are more likely to throw a fit, gesture hitting the other person, or even end up hitting them for real.

It’s because, for children, “retribution comes before gratitude,” research published in Psychological Science, found. Led by Peter Blake, an associate professor at Boston University College of Arts & Sciences, the study aimed to find out when kids learned to pay back for acts of kindness. He found they reciprocate negative behavior before they do positive behavior.

The researchers picked a sample of 330 children between ages four and eight. In each trial, children were made to play a computer game with four other players, who on screen, were animal cartoons, but were actually controlled by researchers.

In the first trial, all four of the other players got a sticker, but the child got none. Then one of the four players was dictated to give it to the child. In the next part of the same experiment, it was the child’s turn to get the sticker while the four players got none.

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In the next trial, the difference was that one of the four players had to steal the sticker from the child. And then, the child was also given a chance to steal it from any of the four players.

After conducting a series of such experiments, researchers realized that kids reacted immediately against those who stole their stickers, even stealing it from the same person who stole theirs, but didn’t show any gratitude towards those who gave them the sticker when they were instructed to do so.

“We were really puzzled by it,” Blake said in a press release. He said it may just be possible that kids indulge in revenge as a defensive move to be able to save themselves from being troubled in the future. “Kids aren’t out to get people. They’re sending a signal to the person, but also to the broader world that ‘I’m not a sucker,’” he added. Another possible reason, Blake said, could be that young children expect compassionate behavior, so any unfriendly behavior may end up evoking a stronger response as compared to friendly behavior.

If parents want to teach their kids to be more thankful, all they have to do is tell them bedtime stories that teach them gratitude, according to the researchers. The idea was proposed by Blake’s colleague, Jingshi Hu, who observed that because kids in China learned about gratitude through proverbs and stories, they were more likely to show such behavior earlier than American children. Hu was right; researchers told the kids participating in the study stories that taught them to be thankful. These kids were more likely to show gratitude to those who gave them stickers and this behavior only grew stronger with age, Blake noticed.

But, parents shouldn’t be worried about their kids’ revengeful behavior because they all “evolve for a reason.” Giving an example, Blake says in the press release, “If someone steals your lunch money every day, you should do something about it. In primate society, some monkeys or apes get harassed, and that can have devastating effects — they can die in the wild. As far as evolution goes, it’s definitely critical that you stand up for yourself.”