Howard Jacobson: Our faith in human nature disappears when the first instinct is to assume the worst

I accept that the default position should be to think the best. That's how we operate socially, isn't it?

So is this Act IV or Act V in the Tragedy of the McCanns? The latest word is that the case against them has suffered a setback, a Portuguese prosecutor saying the police have unearthed no new evidence. Accordingly, the pictures of the couple we found sinister yesterday look entirely blameless again today. There's no knowing how they'll look tomorrow.

The fact that we now measure the fortunes of the McCanns not by the news or no news of their missing child, but by how innocent or guilty the latest findings make them look, shows how far we've come from those almost innocent days of collecting boxes and yellow ribbons. But then this was never going to be a late-Shakespearean romance where the lost child is miraculously recovered; there was always too much culpability in the air for that.

At first I thought we were in the world of Little Eyolf, Ibsen's fantasia of sexual guilt which tells of a little boy who is paralysed as a result of falling off a table while his parents are too busy making love to notice him. The McCanns weren't having sex when Madeleine went missing, but they were out having a good time, which is the next worst thing. Then the tale turned darker still. Most recently we've been talking Medea.

Or at least that's the play some members of the Chorus think they're in. That others insist otherwise proves or disproves nothing. This is what Choruses are for: to contest the status of the play and make dramatic life out of not knowing what to think.

So voluble has the Chorus been these past few weeks that we now have a play within a play. A Morality Play in which the McCanns haven't even needed to be on stage. Enter, stage left, in elevated boots and tragicomic masks, Cynicism and Suspicion; enter, stage right, and similarly accoutred, Sentimentality and Trust. In this allegory we have definitely arrived at Act V, feeling extremely bad about ourselves for thinking extremely ill of other people.

What has happened to our basic decencies, our most conscientious commentators have been asking. Where has faith in human nature gone? What are we reduced to if we assume the worst before we assume the best? I am sympathetic to this view while being sympathetic to its opposite. What are we doing assuming the best when experience has taught us to assume the worst?

I accept that the default position should always be to think the best. That's how we operate socially, isn't it?We work on the assumption that each new person we meet is probably not an abductor or a murderer. Except we don't. If we have children and must entrust them to someone, we are careful to assess the likelihood of that someone being a psychopath or not. We might not pay for a private investigator to tail him, but we scrutinise his face, we judge his character, we ask around, we take up references. In other words, the default position is mistrust.

And when we haven't mistrusted sufficiently and something goes terribly wrong, we are rightly judged to have been irresponsible. Or – which is no better – gullible. In the world which morality governs, isn't gullibility as great a sin as wariness? Don't we fail as moral beings if we aren't suspicious of other people enough?

I recall discussions I used to have as a student about Duncan's famous musings on the defection of the once loyal Cawdor – "There's no art / To find the mind's construction in the face: / He was a gentleman on whom I built / An absolute trust." Which are promptly followed by the chilling stage direction, Enter Macbeth.

Duncan was a fool to his trustingness, some of us thought, almost to the point of being what D H Lawrence called – and we were all Lawrentians then – a "murderee". Even without Macbeth's omen-laden entrance, Duncan stands condemned by his own architectural metaphor. For if the face is an unreliable structure, if it will not support a true account of character – and Duncan is either an exceptionally bad reader of character, or he's exceptionally unlucky – then it's bad building practice to go erecting trust on it yourself. Yet having burnt his fingers in the building industry with Cawdor once, he is determined to do it again with Macbeth.

Others thought Duncan was only doing all any of us can ever do. Where there's nothing to go on, we have to make a leap of faith; otherwise we will wear our lives out suspecting everybody and trusting no one. That there are Macbeths in the world is a tragic fact, but not one we can prepare for.

Myself, I think there is sometimes an art to find the mind's construction in the face, though, like all arts, it is seldom practised well. The common assumption we make, for example, as to what true grief looks like is for ever leading us astray. We didn't care for how Lindy Chamberlain held her face, and therefore we knew the dingo couldn't have taken her baby. (In that instance we'd have done better examining the facial expressions of the dingo.) And we have been making similar judgements about Mrs McCann, as though we know in every instance the lineaments of sorrow.

Only watch an episode of The Jerry Springer Show or something similar, however, where a person with a bone through his nose, a mouth gnarled with more malevolence than a rottweiler's, and eyes that burn blacker than all the fires of hell, is hauled in from backstage and charged with breaking some sad sack's heart because he told her that he loved her, wanted her to have his baby which he'd cherish to his dying breath, and would never look at another woman again. Build an absolute truth on such a gentleman and you get what you deserve.

So, no, while I accept it's hell for the McCanns, I don't think we betray our humanity by suspecting them. Nor will we be retrospectively wrong to have suspected them – wrong in principle, that is – when, as one hopes for them, they are finally exonerated. We don't – if I may put it like this – become wrong just because we weren't right. The practice of suspicion is as necessary to a life intelligently led as the practice of trust. We might have a soft spot for tricksters and deceivers in literature, but we judge harshly, even as we lament their fate, those who don't see through them. To keep your wits about you is a moral imperative, whether you're a lover, a policeman, or just a reader of a newspaper.