A few years ago, when Michael Donovan, the uncle of Karen Matthews' ex-partner, was employed as a delivery driver, his boss sent him out to put £20 of diesel in the van. Later, when Donovan had returned, his colleagues were bemused to see him driving back and forth in front of the company's premises. He explained to them that he had only been able to fit £18.48 of fuel into the tank, so he was driving around until he could squeeze in the other £1.52 of diesel.

So here is one explanation of the dreadful crime by which Shannon Matthews was drugged and kidnapped by her own mother - the perpetrators were simply stupid. Hearing the details of the crime described to the court, it was hard not to conclude that here were two inadequates engaged in a deluded act. Their idea that they might receive £50,000 without being suspected was breathtaking not merely in its evil, but also in its foolishness.

Then there is another explanation. Read Andrew Norfolk's vivid account of life in the grim estate where the Matthews family live. The relentlessly depressing stories of dirt and depression and chaotic lives strike hard. There were so many people in the family that the police did not have the resources to interview them all. When the computer of Karen Matthews's boyfriend at the time, Craig Meehan, was confiscated by police they found child pornography. Brought up in these streets what chance did Shannon stand? Living in these conditions, how surprising was even such a depraved crime as the one committed against her?

Or try a third explanation. Karen Matthews was living without work, or the prospect of work, on £400 of benefits every week. How surprising is it that she began to think that money could be gained without doing anything in return? Without anything constructive to do with her life, is it really so bewildering that her thoughts, and those of Donovan, turned to crime?

Each of these explanations has a sort of plausibility. But each is inadequate. There are many people who struggle to cope with modern life and who lack the standard intellectual tools, yet still are able to make the basic distinction between right and wrong. Great intelligence is not required to prevent you from drugging and kidnapping your own daughter, or tethering the girl while her tearful mother cries crocodile tears for the cameras.

There are also many people who live in poverty and in grim estates, yet still try their best to care for their children and live decent lives. When Karen Matthews started her campaign to find Shannon she had plenty of neighbours to help her. That fact alone suggests that the chaotic lives of Dewsbury Moor, if such they were, do not explain what was done to Shannon.

Nor does living on benefit. For some, dependence on benefit is an unfortunate necessity, for others it has become a way of life when it should not have done. Yet the vast majority of these people don't abuse their children or turn to crime. Not even when they are in the greatest difficulty.

Seeking for an explanation to inexplicable acts of evil is understandable. Trying to understand poverty in modern Britain is laudable. But the crimes of Karen Matthews and Michael Donovan are ultimately theirs, and theirs alone.