Appearances can be deceptive, especially in front of a court of law
Martha Coakley, the prosecuting lawyer, said Louise Woodward's case had raised all sorts of issues, "like who really looks like an abuser. She doesn't present the picture of someone who could kill a child". Deborah Eappen expressed similar thoughts. Yet could someone really look like a child murderer? It sounds preposterous - and yet they were voicing a deep, almost inexpressible, assumption that evil must look evil, that there are physical manifestations of bestiality. And implicit in their thoughts is that with beauty comes innocence.

It is reasonable to ask, therefore, if looks may prejudice the outcome of trials and, in particular, if Louise Woodward's demeanour, appearance and expression was relevant to the way the jury considered her case. During her examination, for instance, she sat in the box with the camera, and all eyes, on her face. The jury and millions of viewers tried to decide her guilt or innocence, straining to match her facial expression to context and to words, trying to see inside her mind. Her expression was described as reserved, and people wondered how this supposedly English characteristic would go down in Massachusetts: it may have done her cause no good.

The cameras roved round the courtroom as the result was read out, picking out the faces of opposing parents, mugging them of their emotions. Our television screens finally showed lawyers, with apparently equal conviction, denying or confirming her guilt from the conflicting evidence and from what they had seen of her performance. We tuned into the drama, the emotional charge heightened and almost dependent on our unpicking the facial expressions displayed in court.

But how sure can we be of our abilities to discern truth and lies from the contours and expressions of the face? The shape of our faces has long been considered a useful guide to character. Aristotle thought large eyebrows a sign of cowardice. In the 18th century Lavater's book on physiognomy went through many reprints; royalty sat at his table. Darwin was nearly refused a berth on The Beagle because his nose was too long. There is, however, no scientific evidence that facial features and character are linked.

If we may disprove physiognomy, what then of facial expressions? Surely we are accurate in reading those? This seems to be the case, but more for the big expressions - happiness, surprise and fear, for example. The recognition of these appears to be innate, instinctive and universal, as Darwin suggested. But the blends of emotions and subtler facial expressions seem far more culturally determined and individual. There is even evidence that most of us are not particularly good at determining whether someone is lying or not from their faces. Politicians and poker players, of course, know this.

Why then do we take so much from faces and their expression? One answer may lie in our past. We evolved, and until comparatively recently lived, in small social groups of 200 or so. Our lives were communal and most of our actions public. We would know a person's character from prolonged social interaction, and lying would, at best, be only a temporary deceit. Now we may pass 1,000 people in a day, seeing but never knowing them. Straining to find meaning in unknown faces we make assumptions, lumping people together.

Paul Ekman, in California, has suggested we are poor at discerning the facial expressions of both truth and lies in strangers and he is exploring ways we can learn to judge falsity on the face. Judges and lawyers may of course, through experience, have gained such knowledge, but what of jurors? Should they be offered courses in facial expression, or warned of the complexity lying within each of our faces? Or would that lead to acting classes for defendants?

Jonathan Cole's 'About Face', a study of the relationship between the face and the self, is published tomorrow by MIT Press at pounds 14.95.