Australasian Science: Australia's authority on science since 1938

Choose Your Friends Wisely

By Tim Olds

Friends, family and co-workers influence our health and happiness to varying extents.

When asked about what one could do to improve one’s fitness, the renowned exercise physiologist Per-Olof Åstrand replied: “Choose your parents wisely”. Now it seems that to improve our health we have to do more than choose our parents: we need to choose our friends because disease (and health) can spread through our real and virtual social networks.

One US study that followed 12,000 people for 32 years found that your risk of becoming obese increases by about 40% if you have a sibling or spouse who becomes obese ( The important factor is social distance (e.g. being the friend of a friend) rather than geographical distance. You won’t become obese if the people next door you never speak to become obese, but if your best friend on the other side of town fattens up, you’re more likely to put on the kilos as well.

The love also has to be mutual. Your risk of obesity is unaffected if your soon-to-be-obese acquaintance considers you a friend but you don’t. If you consider them a friend, but they don’t consider you a friend, your risk goes up by 57%. If you both consider each other friends and your friend becomes obese, your chances of becoming obese nearly treble (171% increased risk). It may be time to find that “unfriend” button on Facebook.

Even death can travel through social networks. A very early study found that your risk of attempting suicide is four times higher if you have a friend who has already done so (

While death and obesity are among the near certainties of life, it stands to reason that if bad things can spread among friends, so can good things. Happiness spreads through social networks like head lice through a kindy class ( Your chances of becoming happier increase if you rub shoulders with people who become happy. If your next-door neighbours become happy, your chances of becoming happy increase by 34%. If a friend living within 1.6 km becomes happy, your chances of becoming happy increase by 25%. The happy rays emitted by siblings (14%) and spouses (8%) are apparently less strong, Newly happy co-workers have no effect at all, so it’s OK to be grumpy at work.

Quitting smoking can also spread from friend to friend ( If your significant other quits, you have a 67% decreased chance of smoking.

Health and disease may run through online networks as well as face-to-face networks. A recent review of the use of online social networks as a vehicle for health interventions by my colleague Dr Carol Maher showed promising results (

More skeptical readers of Australasian Science will doubtless be asking themselves at this point whether all this couldn’t be due to cclike people attracting like. While it’s true that obese people are more likely to befriend other obese people, this doesn’t explain the increased probability of becoming obese.

Of course, it’s possible that people in the same social network share characteristics that incline them to obesity (they may be part of the same baking circle, or enjoy sedentary leisure pursuits like video games). Only a randomised controlled trial would settle this question. We could surreptitiously insert people into social networks – say smokers and non-smokers – and see what the network effects are but I can’t see this going down well with ethics committees.

Some researchers have even exploited social networks to spread health interventions. In Honduras, researchers wanted to introduce a multivitamin supplementation program ( The idea was to spread the program via word of mouth, starting with 5% of villagers. In some villages, researchers randomly selected the initial targets, and in other villages they randomly selected individuals, asked them to name a friend, and then these nominated friends became the initial targets.

While it wouldn’t seem to matter whether you chose a random person or their friend. But it did. Uptake of the multivitamins was significantly higher in the villages where the initial targets were the nominated friends. This interesting oddity arises from the “friendship paradox”: on average your friends will have more friends than you have ( More worryingly, it means that your sexual partner will probably have had more sexual partners than you have had. The friendship paradox arises from a sampling bias, whereby people with a greater number of friends are more likely to number among your friends. Ditto sexual partners: if people sleep around a lot, they’re more likely to be sleeping around with you.

Remember that. There’s nothing wrong with sleeping around — but just make sure they’re not getting fat.

Professor Tim Olds leads the Health and Use of Time Group at the Sansom Institute for Health Research, University of South Australia.