Being an unwanted child is probably a more common experience than you might like to think. Its effects are considerable.

A Czech study identified 220 children whose mothers had given birth to a child they had twice been refused a request to abort. Aged nine, these children were doing markedly worse at school and were more prone to irritability and defensiveness. These problems were still significant when they reached the age of 15. Asked about their mothers' attitude to them, the children claimed she was either 'neglectful' or 'interfering'.

Margaret Thatcher's daughter Carol suffered from being unfavoured, if not actually unwanted. Whereas her twin brother Mark was her mother's favourite, Carol was a dustbin for negativity. She recalls that, 'Mum would say, "Carol those shelves are such a mess," if she visited my flat. Mum has never been keen on anything I have worn: "Ghastly" was a word frequently used.'

As tends to be the case in homes where there is strong hostility to one child, there was little love for either, so that even Mark recalls that although his mother indulged him when she was present, there were, 'only spurts of motherhood'. Carol states that, 'Mum was perfect. There was nothing she couldn't do but she didn't do it with enormous warmth. Mum and me weren't really that close. I don't remember her being physically demonstrative.'

In fact, Margaret was scary: 'As a child, I was frightened of her and later I was conscious of talking to her, knowing her mind was elsewhere. I used to console myself by thinking, "Carol, maybe its because you talk such drivel."'

Carol has a low opinion of herself. She compares her looks unfavourably to her mother's and doubts her attractiveness to men.

'Mum's better raw material than me. I still don't measure up. Unloved is not the word to describe me, but I never felt I made the grade. I always felt I came out second [compared with her brother].' In some families such unfavoured status is made up for by love from the other parent, but Carol reports that, 'There wasn't really a daddy-daughter relationship.'

For such extremely unfavoured children, or ones who are acutely aware that other siblings are getting an unfair share of the parental pie, a lifelong sense of having 'a raw deal' can endure. The enduring animosity between the Freud brothers - the artist Lucian, broadcaster Clement and the lesser-known Stephen (who runs a shop) - may date back to such rivalry for love. I know of it because my mum was at school with them, and recalled that they were extremely rivalrous (she has also never forgiven Lucian for making her horse ill by feeding it oats).

Not that my mum was above creating rivalry herself. We were always told to emulate a well-mannered pair of goody-goody friends - although I suspect my parents would have been horrified if we had actually been like them.