Depression in young children may often go undiagnosed, simply because they can't express themselves in the same way as adults. According to Anna Olek, from the Mental Health Foundation, this may partly explain why very young children's mental ill-health has not been addressed as much as it should have been in the past.

She says: "Part of the problem is that mental illness is so stigmatised that people don't like to think of little children as having such problems."

However, studies have shown that 15pc of pre-school children have a mild mental health problem and 7pc have more severe difficulties that require professional help.

The Scottish Executive recently revealed that, in Scotland alone, an estimated 150 under-fours are being treated for depression.

Luke Reader, a spokesperson for the executive, said: "It can be difficult to diagnose depression in very young children because of their immaturity. It is a sign of their GP's or health visitor's skills that they can detect these children's problems very early on - problems which simply may manifest themselves in the child being withdrawn, or showing a change in behaviour."

Certain children are more at risk than others. Risk factors for depression include learning disabilities, difficulty with communication, certain physical illnesses - particularly chronic or neurological disorders - and low self-esteem.

Risk factors also exist within the family, as the development of both depression and anxiety is heavily influenced by environmental factors. Family breakdown or parental conflict may put children at increased risk of depression, as might inconsistent or unclear discipline or abuse of any kind.

Death and loss, including the loss of friendship, may also deeply affect a child.

Anna Olek agrees that looking for behavioural changes is key to the effective diagnosis of depression in children.

"You're looking at things like perhaps they're not sleeping properly, perhaps they're soiling themselves, perhaps they have nervous tics, perhaps phobias.

"Depression can be manifested by a huge lack of interest in most things, and being very withdrawn."

The presence of conduct disorders such as attention deficit disorder, aggressive or antisocial behaviour, or excessive defiance, may also be a sign that something is wrong.

But how do you know when behavioural problems are serious or just a phase the child is going through? Anna Olek advises parents not to panic, as childhood is a major time of change and children take different lengths of time to adapt to each stage of their lives.

"It's about trying to get the balance right and not trying to pick up on every tiny little thing that your child's done and thinking, `Oh my God, they've got a mental health problem, I need to see a doctor'."

However, if two or more of the behaviour types described here go on for more than a couple of weeks, the Mental Health Foundation recommend that parents arrange a check-up with a health professional.

Luke Reader says: "GPs can ensure that they, health visitors and other health professionals are able to keep an eye on the child's progress and take any necessary steps to prevent more serious problems in later life."

Mental Health Foundation

Young Minds