You and I have almost certainly never met, but what happens to you matters to me – your health, your happiness and your wealth – and vice versa. Mutuality rather than independence is the chief characteristic of human life, whatever we'd like to believe.
Many prefer to see human life as one long competitive struggle for dominance. Philosopher Edmund Burke, Darwin's champion Herbert Spencer (who coined the phrase "survival of the fittest") and Ayn Rand (high priestess of the American idea of rugged individualism) are among those who characterise human life in terms of the struggle between individuals for the spoils of humanity.
Science is increasingly contradicting this view: rather than being a species of arch individualists, we are the social ape. We live in larger, more complex groups than our closest cousins, collaborating with friends and strangers thanks to our nuanced social brain. Indeed, we use other people's brains to navigate the world: to acquire skills and practices, and to access knowledge systems of long-dead strangers. We call this "culture".
We are so inextricably embedded in this world of others that what the people around us do shapes each of us: recent studies (such as Christakis and Fowler's Connected) show how all kinds of things spread through our social connections. If one of my friends gains weight, drinks or smokes, I'm more likely to do so, too. Ditto less serious things such as the clothes we wear and the music we listen to. And these effects seem to work across two or three steps of acquaintance – so even if you and I never meet, what happens to you can touch my life directly. Like it or not, we're in it together. It's mutual.
Mark Earls is author of 'Herd: How to Change Mass Behaviour by Harnessing Our True Nature'. Follow his blog at: http://herd.typepad.com/