Not even on their levels of understanding and empathy. No, your happiness depends on how much money your friends have. If they have less money than you, then you are happier than you would be if they had more money than you. Yes, I know it isn't exactly rocket science. But still, it's nice to have some evidence to back up what we have intuitively felt for years. No matter how rich and successful you may become, it is still not enough if your friends are even richer and more successful. It doesn't matter how much money you make; there will always be a niggling feeling of dissatisfaction if your friends have a higher disposable income than yours.

If, on the other hand, your friends are doing much worse, if their houses are in a less impressive neighbourhood, their children go to cheaper schools, their car is half the size of yours, and their pay packets not nearly as large - well then, you can't help thinking that you must be doing all right.

Sad but true. But then, how could it possibly be otherwise? Peer pressure being what it is, all of us measure our success by the successes (and failures) of our friends. And as long as we are doing a tad better than them, we feel good about ourselves. After all, it's all relative, isn't it? Of course, friends do much more than provide a baseline for comparisons - financial or otherwise.

They influence our thinking, shape our lives, hell, they even determine our shapes. Research shows that if your friends are fat, you are more likely to be overweight. Conversely, if your friends are thin, then the chances are that your weight will remain under control as well.

If you think about it, this makes perfect sense. If your friends are into healthy eating and exercising regularly, you will probably be shamed into doing the same. Nobody wants to be the only blimp in a group of thin people, the target of tired old jokes like "Who's your fat friend?" On the other hand, if your friends gorge on vast quantities of fast food while slumped on the sofa in front of the TV, then you will also feel no embarrassment in doing the same.

And then before you can say Big Mac, all of you will be shopping for elasticised trousers and extra-large T-shirts together. I am sure you must have noticed this in your own lives as well. Go out with a friend who orders a green salad and steamed fish and invariably you will end up ordering a more healthy meal yourself. Go out with someone who gorges on fries and hoovers up the dessert and the chances are that you will stuff your face too. Eating with greedy people triggers your greed impulse as well.

On the contrary, sharing the table with disciplined eaters makes you rein in your appetite. That's how fat friends make you fat while thin friends keep you thin. As if that wasn't enough, friends also have a nasty knack of showing up all our flaws and imperfections. Tall friends make us feel short, thin friends make us feel fat, and fashionable friends make us feel dowdy in comparison.

But it is on much deeper levels that friends really impact your life. For instance, the moral codes that we live by are also reinforced by those who are closest to us. If your friends think nothing of cheating on their spouses - hell, they even boast about it - then you may well be more inclined to give up on marital fidelity. If your friends think nothing of taking or giving bribes, of adopting corrupt practices in their businesses, then it is that much harder for you to stay honest. If your friends routinely lie and cheat about matters large and small, then you are more likely to do so as well. After all, if everybody is doing it, can it really be so bad? (Short answer: yes, it can, but that, as they say, is another column altogether.)

On the other hand, if you move around in a circle where everyone takes pride in being honest, in paying their taxes on time, not being part of the black economy, trying to stay on the right side of the law, then you would probably stay on the straight and narrow as well (if only for fear of being socially ostracised). The truth of the matter is that your friends can help you rein in your worst impulses. Or they can encourage you to give in to your basest appetites. It all depends on what kind of friends you have.