Hi, mum!

Next time a person swears at you in traffic, why not wave giddily at them so they'll spend the rest of the day wondering which acquaintance they had just insulted.

It might also prompt them to consider the great human frailty - that we so often treat strangers in a way we'd never try with our friends.

It's a flaw of our nature on display every morning in traffic queues, as people cut each other off, then mutter in disbelief when strangers do exactly the same to them.

Imagine, however, if we knew everyone in the cars beside us? If the person trying to merge frustratingly late was our mother, or the man we're overtaking dangerously was our best mate or son?

Respect for our own is why your average Aussie doesn't chuck a chip packet out the window of his car on his own street - because the person who'll most likely have to pick it up could be a neighbour, even a family member.

However, on an anonymous freeway, it'll probably be a faceless council worker doing the cleaning, so ... out the window goes the chip packet.

One of the fundamental limitations of our monkey brains seems to be we can only empathise with a finite number of people - usually our friends, family and colleagues - before many of us trail off into apathy and then, too often, outright contempt.

And when you won't delay your passage through the world by three seconds, to allow another car to merge, that's the germ of contempt.

In that moment, where we'll casually put another person's life in possible danger so we can get two metres closer to a red traffic light, you see so much of what makes us human.

Wars, crime, violence, corruption - they all sprout from our willingness to do stuff to strangers we'd never do to our friends.

In Malcolm Gladwell's 2000 book The Tipping Point, he writes of "Dunbar's number", a theory that states the fixed size of our brains sets a cognitive limit to the number of stable social relations we can share.

That number is usually estimated at 150. More than 150 friends and we start to lose track - Hello, Facebook! - which is why organisational units, whether tribal, military or in business, work so well if kept under that figure.

I get the feeling that if a lot of people knew of this "cognitive limit", it'd work as a rationale for them acting rudely in traffic, their face staring rigidly ahead as they burn you, telling themselves "well, it's impossible for my brain to care about any more people".

And maybe it is. It can certainly get overwhelming having celebs, charities and politicians telling us we need to weep for Sudan, the homeless and every one of those kids whom Joseph Kony kidnapped.

But what if our continued evolution is not about letting hundreds of new people into our brains (and hearts), but just the one stranger in front of us?

Instead of being able to empathise with 150 people, maybe we should try 151?

We need let only one person merge on the road - not two or three - but, if everybody did this, perhaps traffic jams would disappear.

Imagine, then, if we applied the same "care" to the one stranger who has to pick up that littered chip packet, the one whose wallet we return with the money intact, or whom we acknowledge was first at the bar?

I know it's nothing new, but "doing unto others as you'd have them do unto you" seems quite a radical sentiment nowadays. It certainly has a ways to go before catching on.

Sam de Brito's latest novel Hello Darkness is in bookstores now. You can follow him on Twitter here. His email address is here.