Pupils ¿catch¿ cleverness from friends: Study finds Children¿s school marks improve if their pals are also doing well If your child has underperforming friends it could drag their grades down Smoking, weight and cheerfulness are also affected by peers You've paid for a private tutor and spent hours helping your child with his homework. But have you vetted his friends? Research suggests that cleverness is contagious. A study found that if pupils" friends were doing better than them at school, their own marks improved over time. Study buddies: peer pressure and oneupmanship among children is thought to boost their grade attainment as they compete to keep up with friends Parents won't be surprised to know that the reverse was also true, with those whose pals were struggling academically starting to do less well themselves. The US researchers said the results suggest that academic success is infectious - or spread through networks of friends. They began by asking 16 and 17-year-old pupils in a school in New York State if they were willing to take part in the study. Those who agreed were given a list of participants and asked to mark each as a friend, best friend, acquaintance or stranger. A year later, the researchers compared the pupils" marks over the preceding 12 months with their social network. This revealed a strong link between friendship and the rise and fall of grades. Not surprisingly, 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 because they are similar to us. This creates 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 their friends" behaviour as the norm and so begin to act in a similar way. This could include the amount of effort put in at school. Peer pressure could also be a factor, in that if a student's friends are doing well, they may want to keep up to save losing face. Similarly, those whose pals have little interest in schoolwork may find it more difficult to buckle down. Another possibility is that when the pupils named their friends, they might have unconsciously added in people that they weren't that close to but they admired. Then, in emulating these people, they worked more, or less, hard. The study, detailed in the journal PLOS ONE is not the first to suggest that physical and character traits are infectious. Doctors have shown the company we keep has a huge effect on our weight, with those with fat friends being almost three times more likely to become obese themselves. Success breeds success: A-Level students with their excellent results celebrating together The 32-year-study of thousands of men and women also revealed that friends influence weight more than family - suggesting friendships are more important than genes in the battle of the bulge. The researchers, from Harvard Medical School, said the phenomenon could not simply be explained by people feeling most comfortable in the company of those of a similar build. Instead, it seems that we take our cues as to what is an acceptable weight from those we like and trust the most. Smoking habits and cheerfulness are also said to be contagious.