Ours had this feeling of four random people thrown together - which, I suppose when you have no shared DNA, is exactly what it is.
Mine were good, loving parents. We were happy. But particularly during my teenage years, I remember the frustration of trying to find common ground. I envied friends whose parents understood them at least some of the time. Mine seemed to look at me, utterly bemused (mind you, I was an awful teenager).
So when I met my birth family when I was 18, you can imagine my joy at finding, for the first time, people who not only looked like, but felt like, me. They, too, were excitable and often spoke before they thought, then worried about it afterwards. Similar things seemed to make them angry, sad or happy. There were other connections - the way my birth father arrived at the conclusions he did, the way particular things made him cringe.
Tragically, my birth mother died when I was a baby, so I would never meet her. But I got to know her parents and four older siblings and, indeed, my birth father and his extended family. It took time to get a true sense of their personalities, but I was struck by this strong likeness.
Later, I started working for an adoption charity as an intermediary in adoption reunions and I noticed how many adopted people even spoke like their birth parents - with hesitancy, very slowly or in a high-pitched voice, for example - having never met them. Even the way they responded to the contact from another person often revealed similarities - you'd get the dramatic types or the quietly dignified types; the ones who sobbed and the ones with long silences.
No prizes, then, for guessing where I sat in the nature/nurture debate during my 20s. In fact, I began to feel something of an authority on the subject when it came up in social conversation. For most people, I explained, it's an abstract concept, impossible to pick apart. For me, it was both real and tested.
But then things changed. Friends increasingly pointed out that, unlike many of them, I'd always been quite responsible with money. I realised I had exactly the same attitude to working, spending and saving as my adopted parents - far more so than in my birth family. In fact, when I stopped to think about it, a lot of my values were the same as theirs.
When I went away for weekends with Mum, I noticed how easily we would chat on most issues - because, I realised, we had a similar outlook on many things. She, meanwhile, has always said I'm like my dad, who died two years ago. "I don't think so," was always my stock response, wondering what planet she was on, but I've come to see that I do have many of his principles, traits and aspirations.
I have children of my own now - Lucy, aged 4, and Sam, 2 - and I've often caught myself (as most of us do in the end) sounding like my own - adopted - mother. Then there are the aspects of my personality that must surely be nurture, in that they are reactions to things that happened in my childhood.
Nancy Verrier, author of The Primal Wound, says many adopted people share an underlying fear of abandonment, for example, due to being separated from their natural mother at birth.
Robert Plomin, professor of behavioural genetics at King's College London's Institute of Psychiatry, says the history of the nature/nurture debate has been as changeable as my own journey of thought.
"Very early on, before World War II, the pendulum swung firmly towards nature," he says. "But then, with the horrors of Nazi Germany and eugenics, it swung very much over to nurture, so that by the 60s, the thinking was that environmentalism was seen as everything and a lot of mother-blaming went on. Now, with adoption and twin studies, the pendulum is somewhere in the middle, with an acceptance that both are significant."
What's most striking about research on twins who are reared apart then meet for the first time, is that they report similarity in mannerisms, he says - for example how they use their hands in speech, how they sit down, how they walk. "If you thought anything was cultural, it would be things like how expressive you are when you talk. Even artistic preferences such as style of dress are often similar. This is the area I'd like to see studied more."
I remember my own maternal grandfather abruptly walking out of the room the first day I met him. He returned, with tears in his eyes, to explain that while he had expected me to look like my birth mother, what he was unprepared for was that my gestures were the same - the way I moved my arms while talking, the way I threw my head back when I laughed.
Some data, Plomin adds, suggests there is even a moderate genetic influence on interests. "The research on identical twins reared apart reveals that interests, even in things like carpentry, may have a hereditary factor."
In 2009, Plomin himself led a study of more than 3700 sets of twins, that found an unshakeable self-belief is in the genes.
"Everyone has assumed self- confidence is a matter of environment," he says. "But our research shows that it is genetically influenced and that self- confidence predicts achievement at school." Children with a greater belief in their own abilities often perform better at school, even if they are actually less intelligent, he explains. "We are not saying that genes are the only factor or that upbringing and environment cannot change things. But there is something genetic in self-confidence which I would think of as a personality trait that would be stable throughout life."
The more precise the character trait, the more likely you are to find a genetic link, reports Simon Moore, leader in psychology at London Metropolitan University. "Sociability, for example, is probably more associated with your father than your mother, while educational-based intelligence is more related to mothers."
But it's important not to get carried away with genes, he warns. "There are five key characteristics that tend to be studied in this context: openness, consciousness, extroversion, agreeableness and neuroticism. When you study these in identical twins, the biggest percentage is for extroversion, but even that's only 27 per cent similarity, suggesting that nurture is more influential than nature. That drops to 16 per cent for agreeableness and the rest have even lower percentages."
But, he says, personality isn't a neat, objective concept. "If you are labelled 'unstable', you are considered someone who should be locked away, but I think the opposite is true. Personality is designed to help you navigate through life's social minefield and it is, therefore, flexible."
You often hear people say, 'I could never hurt anyone' but, of course, they could under the right circumstances. Then there are people who say they are introvert, but if motivated, can be extrovert. There are people who fall into the extremes - cripplingly shy, for instance - but for 95 per cent of the population, they have an adaptable set of characteristics, which change on different days of the week and certainly as we get older. We are different things to different people, too."
Oliver James, psychologist and author of How Not to F*** Them Up, believes genes are even less important. "It's not just that studies showing genetic links point to low heritability, but that the twin studies that make genetic claims are presented as being rock-solid science - but they're nothing of the sort," he says.
When twins look similar, they are treated more similarly than non-identical twins, he claims.
"It's a very obvious point that scientists have conveniently ignored. So I never trusted those figures in the first place. Meanwhile, the classic study for adoptees is the Thomas Bouchard study, but that is also extremely suspect because he refused to let anyone see the data. As for studies of twins reared apart, you'll see that many are in touch with each other before being tested. It's inevitable that they'd have been looking for similarities rather than differences during that time and if we're in a situation where we are looking for similarities, we will find them."
I would have done the same when I was 18, James believes. I can see that, but only to a degree. Even my mum has been struck by the similarities between me and my birth family - similarities that, in all honesty, she'd probably prefer not to exist. Still, James insists that culture, the way parents care for their children and position in the family are by far the most influential.
His last point is important. Debate about the significance of birth order goes right to the heart of the nature versus nurture argument. It all started when Austrian psychiatrist Alfred Adler argued that it can define the way someone deals with life. He identified first-borns as driven and often suffering from a sense of having been "dethroned" by a second child. Younger children, he stated, were hampered by having been more pampered than older siblings. But although it's a view since reiterated by other renowned psychologists, slapping generalised labels on a child is controversial. My own children often take turns at being the "naughty one" or the "diligent one" in a single morning.
For me, what's most interesting is that, for the most part, the debate is becoming less about whether it is nature or nurture and more to do with the interplay between the two. That is, how did my upbringing shape the traits I was born with? And how much of my own children's personalities - Lucy's infinite curiosity, her resilience and independence, for example - are down to me (and others) responding to her innate characteristics? How and why will these pan out the way they will be in the future?
Exactly how this interplay works will require a lot more research, so I'll just have to be patient - unfortunately not one of my strong points.
- INDEPENDENT
