All parents know that cuddling children is a vital part of bringing them up.

But what most parents don't realise is that those cuddles can help children become happy and stable adults, by releasing chemicals in the brain which aid the development of healthy stress-regulating systems.

Without these systems, it can lead to traits such as alcoholism and depression, according to a new book, The Science Of Parenting (Dorling Kindersley, £16.99).

Author and children's mental health expert Margot Sunderland says: "Parenting dramatically affects chemicals in the brain, and the effects are long term."

The child psychotherapist, who drew on more than 800 research studies to write the book, stresses: "There are very clear, proved connections between early stress and depression, alcoholism and all the ills of society in later life."

One of the key chemicals involved, for example, is the amino acid GABA, an anti-anxiety chemical which can also be found in alcohol.

"If you don't get your GABA from your parents, you can turn to alcohol and get it there," explains Sunderland.

High stress is very damaging for the brain's chemical systems, and she warns that controlled crying in babies is a prime example of this.

A baby's brain hasn't yet developed the capacity to calm itself so it needs its parents' help.

If this help doesn't come, as in controlled crying, the brain pathways which enable the child to calm itself don't develop.

"If you're not very careful about comforting them in the first few years of life, you can blight this system," Sutherland warns.

And it's not something you can remedy in the future, as studies show that children who have had little affection when young will not release oxytocin, a stress-relieving hormone, even if they're cuddled at a later stage.

Sunderland says affection is particularly important in the first five years of life and that, of course, includes the terrible twos, when she believes parents should comfort toddlers during tantrums.

This, she explains, is because some tantrums emanate from a child's pain system.

"If a child had a cut hand, you wouldn't say hush. Distress tantrums need to be taken seriously and children need comforting or they become hard-wired to affection.

"Some children seem to get away with it fine if there's not enough affection but in adolescence it puts a huge strain on them and they can go into depression and rage."

And parents also need to reduce their child's stress levels after disciplining them.

"We're not asking for perfect parents," Sutherland says, "but if you shout or scream, repair it. Don't leave the child in a state of shock pick them up, and bring down the state of alarm.