Social media platforms like Twitter amplify expressions of moral outrage over time because users who learn such language gets rewarded with an increased number of "likes" and "shares", a new study suggests.

The study, published in the journal Science Advances, found that users who received more "likes" and "retweets" when they expressed outrage in a tweet were more likely to express outrage in later posts.

"This is the first evidence that some people learn to express more outrage over time because they are rewarded by the basic design of social media," said researcher William Brady from Yale University.

According to the researcher, moral outrage can be a strong force for societal good, motivating punishment for moral transgressions, promoting social cooperation and spurring social change.

It also has a dark side, contributing to the harassment of minority groups, the spread of disinformation and political polarisation.

For the study, the researchers assembled a team that built machine learning software capable of tracking moral outrage in Twitter posts. In observational studies of 12.7 million tweets from 7,331 Twitter users, they used the software to test whether users expressed more outrage over time.

The findings indicated that the incentives of social media platforms like Twitter really do change how people post.

To back up these findings, the researchers conducted controlled behavioural experiments to demonstrate that being rewarded for expressing outrage caused users to increase their expression of outrage over time.

The results also suggest a troubling link to current debates on social media's role in political Polarisation.

The team found that members of politically extreme networks expressed more outrage than members of politically moderate networks. However, members of politically moderate networks were more influenced by social rewards.