They found that in some people, this activity was reduced when the subjects participated in a competition as part of a group, compared with when they competed as individuals.
Those people were more likely to harm their competitors than people who did not
exhibit this decreased brain activity.
EXPLAIN THIS QUOTE PLZ

So basically the people that were in groups in the competition had reduced brain activity (or the activity that they are talking about) compared to the people as individuals in the competition. Individuals we're more likely to harm the competition than the people that showed the reduced activity (groups) Tell me if I'm wrong! Hope it helped