The dynamics of any couple are hard to fathom, but conflict can be deceptively subtle, warns relationship expert Kate Figes.

The photographs were indeed shocking. Charles Saatchi's large hand around his wife Nigella Lawson's throat as they sat having an alfresco lunch at Scott's in Mayfair, London. It's the haunting look of deep fear in Lawson's eyes that suggests this is more than just a "playful tiff", as Saatchi subsequently said, hours before receiving a police caution for assault. Lawson, who has moved out of the family home, temporarily at least, is nowhere to be seen.

The media storm surrounding these photos has highlighted what those helping victims of domestic abuse have known for a long time - that it can affect couples of every social stratum, even seemingly confident, successful women who have the means to leave. Domestic violence is one of the most unreported and misunderstood crimes. Two women a week in the UK are killed by someone they know well. Others live silently in fear for years of what their partner might do to them should they leave.

But perhaps what these pictures prove best is our confusion around what domestic violence actually is. In the past few days there have been mountains of speculation around the Saatchi-Lawson marriage: Saatchi's temperament (he's "explosive") and Lawson's troubled past (her mother would "shout and say 'I'm going to hit you till you cry"') have been cited in an attempt to explain what must surely have been an exception rather than the rule. We don't want to believe otherwise from such a golden couple.

But a celebrity union is no different from any other marriage and just as prone to the wielding of power and control, which is, of course, the substance of most abuse. The black eyes, the woman beaten so badly she is forced to seek refuge with her children in an anonymous safe house, are just the thin end of the wedge.

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Within all relationships there is the potential for abuse because it can be so subtle. Most domestic abuse is emotional or psychological long before it becomes physical, chipping away at the other person's sense of self and self-confidence in small but significant ways. Over time, with enough undermining day after day, one makes the other feel so bad about themselves that they believe it when their partner says that nobody else could possibly want them, or love them like they do.

Victims of abuse are often blamed for everything, shamed or humiliated in public. Their partner makes all the decisions or they find themselves increasingly isolated from family, friends or other sources of support. "It's the insidious level of control, the petty enforcement of rules - anything from how you wrap up the cheese when you put it back in the fridge to how you close the car door," one married woman told me for my book, Couples: The Truth. "And you think this is just a small thing - 'okay, I will do that because it doesn't matter'. Now I can see that what I was giving him was power. That was before he started smashing up the furniture when he got angry and then hitting me."

Domestic abuse can be economic or financial as spouses withhold money or credit cards, make their partner account for every penny spent, or prevent them from having a job or pursuing a career. And abuse can be sexual, not just in the form of marital rape or pressuring someone into sexual practices they would rather avoid, but also by withholding sex.

I will never forget one young woman I interviewed whose husband refused to have sex with her for four years. "He has killed my self-confidence because I feel completely unacknowledged as a woman, and humiliated, too, dressing up for him in sexy underwear and still being rejected. If he had been knocking me about for four years that would be acknowledged as unacceptable controlling behaviour, but this isn't."

Affairs, too, are often a form of abuse, taunting a spouse with the evidence but denying that anything is going on. Instead, accusations of paranoia are hurled back at the victim, dismantling their psyche still further.

Abuse builds when one person in a couple consistently tries to exert that dominance, through intimidation, threats, anger and violence against furniture and walls. There are arguments in every relationship. But there is a fine line between healthy, constructive disagreements and destructive rows full of character assassination and blame.

When a strong man has an anger-management problem, women understandably feel compromised about standing up for themselves. Arguing back could make matters worse. Lawson has been quoted as saying about her marriage: "I'll go quiet when he explodes and then I am a nest of horrible festeringness."

No one can really understand what goes on in another person's relationship. One's own is enough of a mystery. But if I were to turn back the clock seven years and write my two books on relationships again, I would probably structure them differently around the subtleties of abuse because of what I now know.

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What is clear to me is that we find it so hard to understand the very fine line between common relationship difficulties and abusive patterns of behaviour when we are in love with someone and when there are so many other ties that bind us, such as children, reputation, lack of money and not wanting to be alone.

"Why doesn't she just leave?" is a naive statement and one that won't help Lawson, or any other woman in a relationship with an "explosive" man whom she probably still loves.

Our ignorance about abuse is also compounded by the taboos surrounding relationships and family life. We believe our private lives should be kept private. We shouldn't interfere in other people's problems. People took photographs of Saatchi and Lawson but nobody approached the table to ask if they were all right. And it is this hidden nature of family life that makes abuse harder to live with and talk about. For a successful woman, just admitting that there have been abusive situations is tantamount to failure. And so, so shaming.

I wish them both well. Perhaps the most hopeful legacy from this sorry affair will be greater transparency about how common abuse can be. But I also believe that too many people lack the key tools to help them build their relationships from the inside, which in turn allows abuse to flourish. We can't trust everything to love.

* Our Cheating Hearts - Love & Loyalty; Lust & Lies, by Kate Figes, is published by Virago.