You're probably well aware that seeing another person yawn or laugh uncontrollably will set you off to do the same. But there are other more surprising behaviours that have been scientifically found to be contagious, too:
1. Rudeness (particularly at work)
According to research by Trevor Foulk, it takes no more than a single episode of low-intensity negative behaviour to be transferred to those around us. He's likens rudeness to the common cold, in how easy it is to 'catch'. The 'contagion' (rudeness) appears to be unaffected by the relationship between the involved individuals (boss and employee, mother and child, etc), providing stronger evidence that this is an unconscious adoption of anger and hostility rather than a type of mimicking.
There's not much research to clarify why this is more prevalent in work situations, but common sense would suggest it's to do with the high-stress environment and the amount of time you spend in close proximity to co-workers. Foulk also found that previous exposure to rude behaviour makes you more sensitive to it in the future, making you more likely to interpret a situation as rude. Worryingly, further studies showed that the contagious effect of just one rude interaction could last up to an entire week, showing just how important it is to be pleasant to your co-workers.
John O'Doherty, Professor of Psychology at Caltech found that people are more likely to make a financial gamble after witnessing a peer doing it first. Shinsuke Suzuki, co-author of the paper said:
"Both the behavioural and neural responses to taking risks can be changed through passively observing the behaviour of others"
Also, Aislin Mushquash and her team found that couples influence each other's drinking habits so much that you can actually predict one partner's binge drinking based on the other's. Another study published in Alcoholism: Clinical and Experimental Research Journal found that when placed in a social setting, a person's decision on whether to drink alcohol or not was greatly influenced by what their friends drink.
3. Eating habits
There's good and bad news with this one; both healthy and unhealthy eating habits can be passed on to our friends and family. One study found that when looking at over 12,000 people, when a friend becomes obese (as per their BMI), you're 57% more likely to become obese yourself. This increased risk can also be seen to three degrees of separation, meaning you are 10% more likely to become obese if a friend of a friend of a friend is (really!). And if your spouse is obese, it raises your chances by 27%.
This doesn't just apply to the unhealthy, though. When investigating the effectiveness of a weight loss program on 357 overweight patients with Type 2 diabetes, spouses were seen to lose weight at the same rate of the participants. So for anyone looking to change their lifestyle, surround yourself with fit, healthy people and you might start adopting their behaviours.
The main takeaway? Pick your friends wisely.