19 April 2013 Last updated at 13:22 ET

With exam season a little over a month away, Adam Gopnik looks ahead with dread to that painful moment when his children leave the family home.

I want to talk about children leaving home. Not running away from home, though that happens, or kicked out from home. But about the good moment when the time is right and off they go.

As it happens, my own 18-year-old son is getting ready to pack his suitcase and head to college in the fall. When I say pack his suitcase, I really mean it.

When he was born, one of his godfathers came over from London and gave the infant boy a beautiful, antique turn-of-the-century trunk, covered with faded steamer stickers and already filled with judicious presents for his leaving, such as Trumper's extract of limes and a little black book embossed with a gold title announcing that it was for the phone numbers of blondes and brunettes.

"He'll keep putting things he needs in here, and when he leaves home, he'll be ready," said the godfather, whose gift showed him to be both a romantic and a realist. He probably saw the light shining in our eyes at the baby's presence, and knew that he might need a nudge to get out - or rather that we would, to let him.

Well, the day has arrived, almost, and I won't pretend I like it.

The thought of his leaving home is almost unendurable for me. It's partly because we have a kind of all-day radio sports phone-in relationship. The morning usually begins with an exasperated conversation about Chelsea's latest episode of over-spending and the evening usually ends with another about the difficulties of ice hockey's Montreal Canadiens, our two shared sporting obsessions.

And now, I know, that long, continual conversation is ending. Soon, I'll call him on the phone and start: "Hey, do you see what Abramovich…" and he'll cut me short: "Dad, I got to run… Let me call you back?" Two or three days later, he will.

I suspect he will return one Christmas soon with an icy, exquisite, intelligent young woman in black clothes, with a single odd piercing somewhere elegant - ear or nose or lip - who will, when I am almost out of earshot, issue a gentle warning: "Listen, with the wedding toasts - could you make sure your father doesn't get, you know, all boozy and damp and weepy?" My son will nod at the warning.

I am blessed to still have his little sister at home, a 13-year-old who speaks a strange abbreviated Manhattan lingo. "Ily," for instance, means "I love you", which she utters at rapid machine gun-speed from her downturned head, while her thumbs are flashing over the keyboard of her phone, continuing text exchanges with five other 13-year-old girls.

She is like a cross between Lieutenant Uhura on Star Trek and a Gatling gun, spitting out communications with the cosmos. But soon enough will come her message too: "C U ILY".

What I wonder about is why we love our children so asymmetrically, so entirely, knowing that the very best we can hope for is that they will feel about us as we feel about our own parents: that slightly aggrieved mixture of affection, pity, tolerance and forgiveness, with a final soupcon - if we live long enough - of sorrow for our falling away, stumbling and shattered, from the vigour that once was ours.

One theory, popular among the cold-blooded, is that we feel this way only because it's a peculiar feature of our new, smothering middle-class culture. Back in the day, they insist, parents yawned over their kids.

The poor had 10 or 11 children and used them, the myth runs, more or less as the Norwegians used their sled dogs on the way to the South Pole, while the rich hardly saw their children from one year to the next, bumping into them occasionally at a Christmas party. Only the growth of middle-class manners made child love so obsessive.

Perhaps that's so. But then I think of that passage in the first of all Western classics, the Iliad, where Priam of Troy goes to Achilles for the body of his son, Hector:

"Honour the gods, Achilles; pity him.

"Think of your father; I'm more pitiful

"I've suffered what no other mortal has

"I've kissed the hand of one who killed my children."

He spoke, and stirred Achilles' grief to tears

He gently pushed the old man's hand away.

They both remembered; Priam wept for Hector,

Sitting crouched before Achilles' feet.

Achilles mourned his father.

Homer's point, which moved the Greeks and still moves us, was that even in heroic society, the love of parents for their children as children was the strongest bonding emotion of all that humans knew, the one common emotion that could reconcile enemies in grief. Hector, the prince and hero of his people, was also - indeed primarily - Priam's son.

The new and more scientific explanation for the asymmetry is that it is all in our inheritance. Our genes are just using us to make more of them. (That Dawkinsian idea of selfish genes always gives me an image of the galley slaves on a Roman ship, peering and panting out of their little window and then, with a silent nod to each other, deciding where to steer the ship while the captain frets helplessly above.)

Our genes, we're told, force us to sacrifice for our children because they - the genes - want to make more of themselves, and our unequal love for our children is the only way to keep the children healthy enough for long enough to reproduce so that the selfish little buggers - the genes, I mean - can flourish.

The trouble with that explanation is that - as with all genetic explanations of anything involving human love - it restates truths we know already, only in slightly more robotic terms.

An obvious truth - for instance, "women just love guys like Daniel Craig" - becomes "our genes compel women to be attracted to men with a full head of hair, broad shoulders and narrow waists, who are perceived as having high social status." Oh. This does not illuminate our lust, it merely annotates it. It explains the origins but not the intensity of the effect.

Our love for anything cannot be explained by our possession of genes, any more than our love for football can be explained by our possession of feet. It is true that football would be impossible without feet, but the feeling it inspires long ago left feet behind - even Frank Lampard's.

It is not that the big emotions we feel - love or lust or loyalty - are more mystical than their biological origins but exactly that they are far more material, more over-loaded with precise dates and data, associations and allegiances, experiences and memories, days and times.

The mechanism of life may be set in motion by our genes, as the mechanism of football is set in motion by our feet, but the feelings we acquire are unique to our own weird walk through time.

My own best guess about the asymmetry of parental love lies in a metaphor borrowed from the sciences. Merely a metaphor, maybe, but one that - as metaphors can - touches the edge of actuality.

One of the rules of mathematics and physics, as I - a complete non-mathematician - read often in science books, is that when infinity is introduced into a scientific equation it no longer makes sense. All the numbers go blooey when you have one in the equation that doesn't have a beginning or an end.

Parental love, I think, is infinite. I mean this in the most prosaic possible way. Not infinitely good, or infinitely ennobling, or infinitely beautiful. Just infinite. Often, infinitely boring. Occasionally, infinitely exasperating. To other people, always infinitely dull - unless, of course, it involves their own children, when it becomes infinitely necessary.

That's why parents talking about their children can be so tedious - other parents, I mean, not me or you - not because we doubt their love, or the child's charms, but because itemizing infinities is obviously the most boring thing imaginable.

We see this, with heartbreaking clarity, in those people we know, or read about, who continue to love, say, a meth-addicted child. And we think: "Why don't you just give up?" And they look at us blankly and we say: "Oh, yeah. Right."

The joke our genes and our years play on us is to leave us, as parents, forever with this weird column of figures scribbled on our souls, ones that make no sense, no matter how long you squint at them or how hard you try to make them work.

The parental emotion is as simple as a learning to count and as strange as discovering that the series of numbers, the counting, never ends. Our children seem, at least, to travel for light years. We think their suitcases contain the cosmos. Though our story is ending, their story, we choose to think - we can't think otherwise - will go on forever.

When we have children, we introduce infinities into all of our emotional equations. Nothing ever adds up quite the same again.

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