Diya says she was never in any doubt her mother had a favourite child – and that it was not her. Now, with three young children of her own, the 27-year-old thinks it is because she looks like her father, who left when she and her sister were very young.

“I remember my dad coming to my defence once when I was about 12, telling my mum that she couldn’t choose to love one daughter more than the other. That was the last time my mum let him in the house,” she says.

Diya, who has “spent a lot of time with counsellors over the years, talking about my mother”, still struggles to accept the difference in how she and her sister, just a year older, were treated by her mother. “It persists to this day,” she says. “My mother spent £6,000 on my sister’s wedding and did a load of the organisation for her. I got married recently. She paid for half of my wedding dress and finally gave me a present many weeks later. She didn’t get involved in the wedding arrangements; she couldn’t find any time for me at all.”

Diya’s big fear is seeing the favouritism pass down the generations. “My mother has let my kids down a few times already – cancelling at the last minute on their birthdays – but now my sister is pregnant,” she says. “I’m just waiting to see if her baby gets more attention than my kids. If I see any signs of favouritism there, too, then the relationship between my mum and me will be finished for good. I’m not having my children feel the pain I did.”

She is not alone. In a recent study, 85% of respondents believed that their mothers had a favourite among their siblings. The finding chimes with many years of research about parental favouritism, which has found that many parents admit to having a favourite child. Dig a little deeper, though, and it turns out that most favouritism has less to do with love and more to do with like: the same parents say that they love their children equally, but that one child’s personality resonates more with them than those of their siblings.

But Prof Helen Dent, who has worked with families where one child is made the scapegoat, usually of their parents’ unresolved emotional baggage, says it can cause serious problems. “It’s highly damaging to a child when they are the only one their parents will not care for,” she says. “These children are likely to fare much less well because they internalise all the bad things being said and done to them. This can have horrific effects on their self-esteem.”

Sara grew up knowing – and accepting – that she was the black sheep of her family; that her sisters were loved and cherished in a way that she was not. “There was no question about it: from a young age, I was the least favourite of my parents’ three children,” she says. “I remember being jealous of my sisters when I was very young, but then I became resigned to feeling that I was an intruder, on the outside of my own family.”

Growing up in an outwardly happy family – her mother was a midwife, her father a doctor – it was only later that Sara understood the hidden dynamic: “I found out when I was 11 that I was illegitimate,” she says. “My mother was pregnant – and had been deserted – when she met the medical student she went on to marry. In their minds, I wasn’t a ‘doctor’s daughter’ like my sisters: I was just some random man’s offspring.”

Sara, now in her late 50s, is in no doubt that her parents loved and liked her less than her siblings. “When I started school at five, I was left alone. And I mean alone: Mum was proud of telling people that she could stay at work until late because I came home, cooked my supper and often put myself to bed.”

When her sisters came along, they had very different childhoods: “They were never left alone. They were favoured emotionally and financially. My sister was bought a record player for her ninth birthday, for example, and I got nothing. Later, they both got driving lessons. Not me.”

The impact of such early ill-treatment has affected Sara throughout her life, but the favouritism scarred her sisters, too: “My sisters feel guilty because of the attention they got from my parents. And because of the way they were treated as children, they now pretty much feel they can do no wrong and are better than those around them. They’ve both had very bad relationships with men as a result.”

Sara is sure that it is worse being in a family with a favourite than it is to be in an unloving household. “It’s not just that the ‘unfavoured’ child feels unloved. Their siblings feel lots of pressure on them to be perfect, and to be the duplicate of the parents all the time. They grow up with this massive, underlying fear that their parents’ love could be withdrawn from them as it has been from their sibling.”

Dent agrees. “Siblings will feel gleeful and relieved that they’re getting the approval, but they will also, consciously or subconsciously, feel guilt and a lack of safety because, if their parents’ love isn’t unconditional, who knows what may happen to them next?”

Margaret, 46, started counselling 18 months ago and came to realise how scarred she was by the fact her sister was so obviously her parents’ favourite. But she also realised that her relationship with her sister had broken down a few years before largely because of emotional wounds carried by them both since childhood.

“I think that it is her relationship with me that has been affected, rather than mine with her,” she says. “My sister grew up with the upper hand, getting all of my parents’ praise. We have tried to be close as adults, but it doesn’t work. I suspect she feels guilty about how I was treated, but also – because she grew up walking this rose-strewn path that my parents laid before her – she wasn’t prepared for the fact that real life is tough. I’ve ended up doing better than her professionally, and she finds that very hard.”

Thanks to counselling, Margaret recently began unravelling her childhood. “I hadn’t realised that I’d internalised the message that I wasn’t good enough,” she says. “My mental wellbeing has been badly affected. It has made me push myself to the limit on every occasion; I frequently make myself ill with excessive determination to push things through. I feel I have to constantly prove myself. I never ask for support or help.”

Margaret, however, has a close and loving relationship with her daughter, who is 11. “I have made sure that I’m a really good friend to my daughter, as well as being her mother,” she says. “I have very consciously made her the centre of my world – the exact opposite of where my parents put me in the family.”

Some names have been changed