02/10/2012 12:35 EST | Updated 04/11/2012 05:12 EDT

Adultery: Someone Always Loses

Of the 1,154 past or present human societies, nearly 1,000 have permitted a man to have more than one wife.

Life-long fidelity is a Judeo-Christian invention, which likely came about for economic reasons: a way to guarantee paternity of offspring for men with property to bequeath and ensure financial stability for women embarking on the long child-rearing years. Eventually, marriage became the precursor to sex (at least in theory).

Nowadays, with modern contraception and larger, more anonymous communities, infidelity is easy. Among !Kung San village, two per cent of kids result from cuckoldry; in some contemporary urban neighbourhoods the figure is five per cent to 20 per cent.

Why is adultery so common? There's the sexual rush fuelled by the secrecy imperative. And the excitement of a new partner brings us back in a temporal slight of hand to previous emotional times, and so the tryst makes us feel young.

The contemporary pathos is also fed by the pervasive importance of sex -- from cooking shows to comic books -- and the belief that good, regular sex is our right. Added to this is the emotional comfort of sex: it is the one time when we are most assuredly living in the now, the most present.

This is the backdrop. There are a million personal variations on the reasons for having affairs, but usually it boils down to two things. First, we think something is genuinely amiss with the home relationship and justify the clandestine relationship as a way to fulfill needs that aren't met in the marriage; second, and more common (although few have the self-knowledge to see it), something is missing in ourselves, be it self-esteem, security, or the capacity for contentment, and we look for it in someone else. Remember: One must be well to love well.

Regardless of the impetus, adultery is fraught with peril.

Clandestine betrayal -- and that's what it is, make no mistake about it -- is nasty. Once you have broken that promise, that vow of faithfulness, there is a terrible sense of loss, no matter how you dress it up. In addition to the betrayal of your shared intimacy, the lying takes a toll. Lying about the where and with whom. Taking furtive showers in the secret lover's bathroom, trying not to get your hair wet. And each lie adds a layer of deceit until you hardly know what to say to your partner anymore.

Then something happens. You may get caught and have to contend with the fallout from a battered ego and lost trust. You may face a gradual, painful and long breakup. Or your partner may kick you out, physically and/or psychologically. Affairs are poignant tragedies if they lead to the dissolution of an arrangement that's good for raising children. If it's between adults, well, it's your call.

But before you start, appreciate that there is a real risk that it will destroy your primary relationship. Decide whether it's worth it. Consider what you have to lose -- socially, financially, and emotionally -- because someone always loses in an extramarital affair.

Barbara Sibbald is a two-time novelist, editor at a leading health journal, and an award-winning freelance journalist. The above is an excerpt from The Book of Love: Guidance in Affairs of the Heart, a novel (General Store Publishing House).