The fear of The Other is not new. It’s hard-wired, like the fight-or-flight response and our ability to detect movement far away to the left or right in the corner of our eyes. These are defence mechanisms, selected by evolution, that help explain why we dominate the planet with our agriculture, architecture and alopecia.

The irony is, we must embrace The Other to progress as a civilisation. We must subdue our most fundamental, instinctive reactions if we are to move forward. Hiding your family in a cave has not been a viable strategy for the continuation of Homo sapiens since the glaciers receded, and it may be what did for our cousins Homo neanderthalensis and Homo floresiensis, the Hagrids and the Hobbits.

If you want to make friends, start a family, build a business, provide a service, learn new skills, do virtually anything, you must dial down the fear of The Other and dial up the social. This starts with tribes. Tribes are a social construct, a group acting as a buffer between family and The Other. Tribes are the familiar: The Known Other, in a way.

In the distant past before even colour television, we each might have belonged to a single tribe. Those in the same village or settlement or cave, or gathered around the same fire. There are still some in western society who never leave their home town and the comfort of their tribe. Today we belong to several tribes at once. The tribe of work colleagues, the tribe of pub mates, the tribe of gay people, the tribe of people who stayed at that hotel that week and ended up in that bar together every night, the tribe of every online forum in which we participate, the tribe of our football club, the tribe of our nation. Each tribe carves a circle of people in a Venn diagram with ourself at the common intersection, possibly alone.

And to each tribe you present a different face. At first it is the mask you wore when you were The Other, gaining entry and acceptance to the tribe. Sometimes that mask never slips, but time brings woodworm and, eventually, the last remnants of otherness fade.

The stresses reappear when two separate tribes meet. You belong to both, but most others belong only to one. At parties, those in common tribes huddle together: workmates there, pub chums there, each avoiding The Other. And you ping-pong between, until alcohol dissolves the borders.

One of life’s most valuable lessons: to bring separate groups together — perhaps two companies have merged, or you’re team-building, or getting married and want the family to meet the future in-laws — apply alcohol. It’s a social lubricant. And it works because it lowers defence mechanisms: you lose your fear of The Other. (I’m generalising, I know. Not everyone needs alcohol to do this.)

Unfortunately this works only when the tribes can see the whites of each other’s eyes, and the glistening bottles behind the bar. It fails to scale. Especially, it fails nationally and internationally.

Britain is a mongrel land, invaded in peace and in war for over two thousand years. We are all immigrants, if you look back far enough. And yet today major political parties still raise the spectre of The Other over immigration, because it triggers our innermost fears.

The effect in the United States is even more pronounced. It is a country of immigrants, a country of others, built from waves of migration that overwhelmed the native population in just a few centuries. And yet it is massively fearful of The Other. The tragic events in Boston in the last week demonstrate this all too clearly. A desperation to pin the blame on the not-we, someone with the “wrong” religion, or skin colour, or any other differentiating feature that could be identified. A desperation then, even by those elected and sworn to uphold the Constitution, to deny the arrested Other the rights accorded to the tribe. Here’s US Senator Lindsey Graham (Republican, South Carolina):

The logic goes something like this: members of the tribe don’t act this way; we believe this person did so; therefore, he must not be a member of the tribe. And you see tweets like this, retweeted out of context by, among others, journalists:

Al Qaeda has been talking about the bombings, nudge nudge, wink wink. Of course. Everyone has been talking about the bombings. There’s a thread on a gay forum. There’s a thread on Mumsnet. They don’t mean the suspects are gay mothers. In truth I’d be more suspicious if Al Qaeda forums weren’t talking about the bombings. This is plain and simple fear-mongering.

Fear regresses us. We revert to ape logic, casting a tribe member out onto the hostile savannah for displeasing the alpha. It is not the act of a modern, decent, respectable, human civilisation.

And yet it happens anyway. Because the alternative is to admit that the person arrested is not, after all, The Other. He’s part of the tribe. He’s one of us.