Actress Gwyneth Paltrow has spoken of her "shame" at crying as she accepted an Oscar for Best Actress at last year's Academy Awards.

In a rambling speech accompanied by much weeping Ms Paltrow, 26, thanked a long list of people before collapsing into the arms of film director Steven Spielberg.

But is crying in public really such an indignity?

"Tears are our emotional thermometer. Too many of us deny them, bottle things up and then things get worse," says Ron Bracey, a clinical psychologist.

Crying is a basic function in humans. Tears clear and lubricate the eyes. But why we cry when we are labouring under strong emotions isn't entirely clear.

Harmful chemicals

Some researchers suspect the body secretes harmful toxins when under severe stress. They think these toxins are excreted through the tear ducts.

People suffering from chronic depression find it hard or even impossible to cry, implying that the body's way of removing its own stress has become disabled.

Others find it difficult to cry even if they are not suffering from depression.

Studies show this reluctance comes from early childhood and the way they were brought up. They can often feel a sense of shame at having "given in to their emotions".

Others find it much easier and feel little or no guilt at exposing their emotions in public.

Children find it easier to cry than adults. This is because for children tears are an instrument of control. In fact, in the case of very small children who can't yet talk it's the only way of communicating their need for adult attention.

A feeling of shame after a crying bout is quite common.

"When we cry we are often hijacked by our emotions. In effect they make us do something some of us would not normally do," says Mr Bracey.

"When we regain control of ourselves, it's as if we have to reassert control very consciously and deliberately, and that includes feeling guilty that 'we let ourselves down.'"

Emotional blackmail

Why and how our emotions become hijacked depends on the circumstances.

Grief is a major stress, and most people accept that it's OK to cry at a funeral.

Other causes can be the achievement of a difficult long-term goal, like winning an Oscar.

Or it can be a reaction to failure. Tears among the losing side in important football matches are not uncommon, especially if one player is responsible for that failure.

Paul Gascoigne's tears at the World Cup in 1990 when he was sent off helped turn him into a household name.

A more recent outburst was from Christine Hamilton after she and her husband lost their libel action against Harrods owner Mohamad al-Fayed.

For other people it can be the relief of tension, or perhaps a sense of loss or melancholy which can precipitate the tears. Former Hong Kong governor Chris Patten famously leaked a little as the Union Flag came down for the last time at the hand over ceremony.

"Crying is something natural. [Tears are] not good or bad. They're just the body's way of dumping excess emotion.

"Shame at crying isn't bad either. It's simply part of the body's way of re-establishing control," says Mr Bracey.