Rejection can dramatically reduce a person’s IQ and their ability to reason analytically, while increasing their aggression, according to new research.

“It’s been known for a long time that rejected kids tend to be more violent and aggressive,” says Roy Baumeister of the Case Western Reserve University in Ohio, who led the work. “But we’ve found that randomly assigning students to rejection experiences can lower their IQ scores and make them aggressive.”

Baumeister’s team used two separate procedures to investigate the effects of rejection. In the first, a group of strangers met, got to know each other, and then separated. Each individual was asked to list which two other people they would like to work with on a task. They were then told they had been chosen by none or all of the others.

In the second, people taking a personality test were given false feedback, telling them they would end up alone in life or surrounded by friends and family.

Aggression scores increased in the rejected groups. But the IQ scores also immediately dropped by about 25 per cent, and their analytical reasoning scores dropped by 30 per cent.

“These are very big effects - the biggest I’ve got in 25 years of research,” says Baumeister. “This tells us a lot about human nature. People really seem designed to get along with others, and when you’re excluded, this has significant effects.”

Baumeister thinks rejection interferes with a person’s self-control. “To live in society, people have to have an inner mechanism that regulates their behaviour. Rejection defeats the purpose of this, and people become impulsive and self-destructive. You have to use self-control to analyse a problem in an IQ test, for example - and instead, you behave impulsively.”