Friendship may be priceless  but that doesnt stop us splashing out on our closest pals.

London - You’ve paid for a private tutor and spent hours helping your child with his homework. But have you vetted his friends?

It could be the key to good grades, because research suggests cleverness is contagious.

A study found that if pupils’ friends were doing better than them at school, their own marks improved over time.

Parents won’t be surprised to hear that the reverse was also true, with those whose friends were struggling academically starting to do less well themselves.

The US researchers said the results, published in the journal PLOS ONE, suggest academic success is infectious.

They asked 16 and 17-year-old pupils in a school in New York state to mark other participants as a friend, best friend, acquaintance or stranger. A year later the researchers compared the pupils’ marks for 12 months with their social network.

An acquaintance’s success or failure had less impact than a friend’s.

However, mere friends were more influential than those listed as best friends.Researcher Hiroki Sayama, of the State University of New York, said that may be because we choose close friends who are already similar to us, leaving less room for change.

Dr Sayama believes there are several possible explanations for why cleverness, or lack of it, spreads between friends.

One possibility is that people view friends’ behaviour as the norm and begin to act similarly.

Peer pressure is also thought to be a factor, said Dr Sayama. - Daily Mail