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According to research from Cornell University, mothers who cast their offspring as golden children who can do no wrong, or black sheep who can do no right, set them up for psychological problems and sibling conflicts that can persist through life.

It was well past midnight, but Emma Swanston was hunched over her computer in her dressing grown in the dark, shivering not from cold but from shock. That afternoon, she had had a vitriolic row with her mother Pauline over her continued snipes at the way she brought up her two sons.

She had been lying in bed, mulling over the bitter exchanges, which had included her mother’s claim that Emma had always been “difficult”, when she remembered something: “A day earlier, Mum had used my laptop to check her emails. It dawned on me that she probably hadn’t logged out.”

What Emma, 42, a graphic designer from west London, saw next explained why she had felt like an outsider in her own family for as long as she could recall. “There were lots of emails to my sister Elaine, going back years, saying what a pain I was, how I was mentally unstable and how sorry they felt for my husband and children — and what a brilliant daughter and mum Elaine was by comparison.”

Elaine lapped it up. “I had always suspected it, but until that moment I had not realised that my position as the least favourite daughter was set in stone, and how much my golden-child sister, with whom I thought I got on OK, was profiting from it. I will never be able to shake off that terrible sense of betrayal.”

This week, a study of 384 families by the University of California, published in the Journal of Family Psychology, found that 74 per cent of mothers said they had a favourite child. From time to time, every parent has probably felt a little more loving towards the child who is being best-behaved — or been more protective of a younger one. And what child hasn’t complained that their siblings get preferential treatment?

Now, a growing number of studies are focusing on the lasting impact on children who become stuck in this family dynamic for good. According to research from Cornell University, mothers who cast their offspring as golden children who can do no wrong, or black sheep who can do no right, set them up for psychological problems and sibling conflicts that can persist through life.

Dr Karyl McBride, author of Will I Ever Be Good Enough? Healing the Daughters of Narcissistic Mothers, believes while boys and girls both suffer emotional disruption, the damage can be particularly insidious when a parent plays favourites with a daughter. “A mother is her daughter’s primary role model for developing as an individual, lover, wife, mother and friend,” she says. “[Not being the chosen one] can lead to feelings of profound self-doubt, best summarised by the words: ‘If my own mother likes me less, won’t everyone feel the same?’?”

She believes narcissistic mothers were often starved of maternal love themselves and shore up their self-worth and feeling of control by playing divide-and-rule among their children.

Typically, experts say this means that one of them — probably the one who makes the fewest emotional demands or reflects best on them — becomes the favourite. They will be elevated at the expense of other children in the family, and often collude with the mother against the others, in order to stay safe. “It’s a pattern so often repeated that it’s textbook,” says Emma, who has since broken off contact with both her mother and sister and joined a monthly support group for daughters of narcissistic mothers, where she has seen the pattern again and again.

“I was quite a morose, quiet child, ungainly and awkward, while my sister, who was three years younger, was an altogether sunnier personality. But rather than trying to help, my mother cast me as the family ‘pain in the arse’ and my sister as the daughter who could do no wrong. “It served my sister because she could feel superior and safe. The more I objected to my stereotype, the more I lived up to my reputation for being difficult. I didn’t realise how badly I was trapped in this toxic family dynamic.” Though she is now a happily married mother, Emma says she is still living with the emotional aftermath.

“I suffer from terrible impostor syndrome and workaholism as I try to shore up my sense of self-worth and prove I am not the person they think I am. I still worry no one will like me. After all, if your own mother didn’t like you from the start, why would anyone?”

For Kate Longman, 30, from York, her mother Kay was always explicit that she was the less favoured child. “She’d openly say in front of me and my younger brother Jonathan, ‘Ooh, give me a houseful of boys any time. Girls are so sly and manipulative.’ While he got pocket money, I had to earn mine doing a paper round. When I got into trouble at school, I was the troublemaker. When he got into trouble, it was the teachers’ fault.”

“As a child you don’t know anything else, so it’s hard to make a sense of that sort of unfairness. But it’s still profoundly damaging. Why did she do it? I think she was raised like that, as one of several girls herself, and this kind of toxicity can be passed on through the generations. I never knew my father, so there was no one to tell her how wrong it was,” she adds.

Joan Bailey has just turned 60. But the retired producer from Worthing, East Sussex, has only recently woken up to the legacy of a childhood in which her mother Elizabeth played a blatant game of divide-and-rule between her and her brother. “She identified me as the cause of the all the problems in family, telling me I’d been an attention-seeker since I was a baby. On my wedding day, she took my husband to one side and said, ‘Best of luck, you are going to need it’, before telling him what a dreadful person I was.”

Even now, she finds it difficult to silence her mother’s voice: “It’s in my head, critical of everything I do. I don’t think I ever lived up to my potential — it always held me back.”

For family psychotherapist Miriam Chachamu, author of How to Calm a Challenging Child, such extreme behaviour is exhibited only by the most narcissistic parents. “Many parents of previous generations did not hesitate to say if they had favourites. Now we know that such parenting can be toxic, it has been swept under the carpet.

But some parents still experience preferences for one child, and they need to acknowledge this so that the issue can be addressed. “As a parent, it does not mean there is something wrong with you. There are times when it’s easy to feel closer to a child who is easier to manage. Rather than shaming ourselves, it’s better to ask if we are unwittingly contributing to this dynamic.”

As for unflavoured children who have grown into aggrieved adults? “If siblings can have honest conversations with one another and their parents,” she suggests, “it’s never too late to heal old hurts.” Some names have been changed