I was born 22 months after my brother, Jeff – almost two years in which he was the glorious, sole and undisputed recipient of my parents' affection. Then one day in January 1956, his mother vanished entirely for three months – confined to hospital with me, a sickly, ailing child.

Jeff was removed from his home and sent to live with relatives. My father, having no car, also barely saw him during this period. When he returned to his own house, he was greeted by a mother who was taking Largactil, a powerful tranquilliser. In her arms was a child who had just come through a trauma that had taken him very close to death. Born with an infant form of cancer, Wilms' tumour, a cancer found in children under five, one kidney had been removed – but I had clung tenaciously to life.

There had been other complications in the birth and I had a strange metal cage on my face to protect my cleft lip that gaped repulsively. Who knows what this vision must have looked like to a two-year-old child? Who knows how he must have felt at seeing his mother return, drugged and carrying a hideous usurper?

But these questions were not asked then. My mother recovered, apparently, as I did, as Jeff did. Life returned to "normal". My parents assumed that no real damage had been done.

Maybe it's true. But I have always wanted to know why I had depression – during much of my later life I have blamed heredity: my mother killed herself in 1988, and my uncle and grandfather had depression. I have blamed the trauma of my birth. I have blamed myself. But it didn't occur to me until a few years ago to blame my brother.

Perhaps "blame" isn't right. "To ascribe as a cause" might be more accurate. No one could blame a 22-month-old for feeling resentful at an arrival such as mine. No one could be surprised that that child might have been damaged by such an incident either.

But these possibilities are not given much credence in my family, neither by my father, Jack – now 86 – nor my two brothers (James, my younger brother, is 13 years my junior). Their view is that children "get over" such things, that conflict between siblings is normal and even healthy.

Again, maybe it's true. But there is an alternative version of events. That the trauma of losing his mother for three months at such a crucial developmental age not only permanently alienated Jeff from me during our childhood, but also alienated Jeff from his mother. Was it a coincidence that he started travelling the world at the age of 17, and settled in America in the early 80s? Was my mother's complaint that he never really talked to her on the occasions he returned to England simply the normal behaviour of a restless young man?

Was the fact that my parents were not invited to either of his two weddings nothing more than a matter of convenience, as he claims? Was the fact that he and I only truly became reconciled after my mother's death a coincidence?

Again, maybe. But when I started writing a non-fiction book on the nature of sibling rivalry, Under the Same Stars, in 2008 – later to become a novel – I felt impelled to get a clearer idea of what had really happened between us.

For Jeff, all my theories about damage and trauma were simply over-intellectualising or attention-seeking. I was inclined to agree with him because I had long ago learned to be ashamed of my restless probing into our family past while everyone else was perfectly satisfied with it as it was.

The fact that I remembered being miserable at his hands, that he was not only hostile but also supremely indifferent to me all through our childhood, seemed to Jeff typically fanciful and melodramatic. The idea that he suffered in any way made no sense to him. As far as he was concerned, his childhood was perfectly happy.

I cannot remember playing with him, laughing with him, sharing with him or ever being happy in his company. I did remember the time he held me under the bedclothes for so long that I thought I was going to die. Even now I become short of breath in an enclosed space.

I also remember the fight we had at my mother's funeral – our father had to pull us apart. And I remember lying on the floor with a big gash on my face after I fell off the top bunk when Jeff and I had been roughhousing. Did he push me off? I can't be sure. Jeff can't remember any of the above events. He thinks I may have made them up.

The question at the heart of my book was, did he buy his happiness at the price of mine? Were his psychological defences – hostility and indifference – an assault on my sense of self, an assault that finally left me with mental illness? Or was I just casting myself, again, as "a victim".

Dorothy Rowe is the author of a book on sibling rivalry, My Dearest Enemy, My Dangerous Friend. She is not surprised my brother remembers a completely different childhood to me, because "each sibling tries to invalidate the other – through memory. When you are children you fight for your parents' attention. When you are adults, you fight over memories."

Sibling relationships have traditionally been the poor relation of parental and peer relationships in developmental psychology. As Rowe points out, sibling relationships are "as various as snowflakes", which is perhaps why developmental psychologists have been reluctant to study them.

The developmental psychologist Judy Dunn, author of Separate Lives: Why Siblings are so Different, agrees that the role of siblings in developmental psychology has been neglected: "It's a very intense emotional relationship and because siblings know each other so well, the possibility of savage and clever teasing is very high. And this starts very early – even in children under two. The competition for the attention and approval of parents goes on till the day they die."

Dunn points out that you interreact with your siblings much more than with your parents – so why should it be somehow less of an influence?

Siblings represent a kind of negative personality development which is called "de-identification". In simple terms, this means, typically, you look at your brother or sister and decide you don't want to be like them, so you can carve out your own identity. This is why children, who after all share half their genes, are often so different.

I tell Dunn the story of my own birth and Jeff's position of abandonment at the time. "Extraordinary," she says. "I'm really astonished that the hospital authorities let that happen. Twenty two months is an incredibly interesting and vulnerable age. Your brother must have had very mixed feelings towards your mother when she returned. Eighty per cent of children act out against the mother when a new child is born, even under normal circumstances. What happened to your brother sounds like a very serious trauma – off the scale for most children."

To see if any kind of consensus existed on the subject, I spoke to Juliet Mitchell, of Cambridge University, author of Siblings: Sex and Violence. She agrees that the power of sibling relationships is underestimated, and points out that, for instance, during wartime evacuations it was consistently siblings that children missed most, not parents.

"Yours is a very rejecting model," she says. "It must have been pretty awful for him to come back and find his mother with a baby. An impact would be absolutely inevitable. How it's handled and his capacity to do what he did with it would be totally different according to the individual and the circumstances.

"It would be normal that he would want to get rid of you, because he was the normal, omnipotent baby. He got to be the 'big boy' who had the edges knocked off him. And he's got to get on with his life, which he did, including going to find a new country for himself. What he did in being taken to his uncle's – he repeated that in going to America. He knows in his psychological bloodstream that he can survive in another world."

I tell her about my feelings of Jeff's painful indifference to me when we were children. "He got rid of you. To me the core of it is, kill or be killed. He would have felt killed by not existing, by being sent away. He was the golden boy of the family, he has been psychologically killed. What do you do to survive? You kill."

I tell her my theory that he built his emotional wellbeing at the expense of mine. "He quite likely did. He's a survivor. And his natural response would have been to turn his back on you. He wanted to attack you and your mother."

I mention that he did not invite my parents to his weddings. "He was probably very angry with his mother. And he would have gradually built a wall against showing any vulnerability. Adults are always trying to get back to that unique, omnipotent position that they once had. The only way you can do that is to destroy your brother – and although they won't physically destroy them, they will symbolically destroy them."

I tell her that the conclusion I drew from my brother's unrelenting lack of approval was that I was bad, that there was something wrong with me. "There was nothing you could do," she says. "Nothing. A child sees himself as agency for everything."

This strikes a chord and reminds me of what Dorothy Rowe had told me: "Small children often discover shame at the hands of their older siblings, who laugh at them and scorn them for being unable to do what they do."

I had spent much of my life feeling ashamed, whereas Jeff said he felt he had regretted nothing he had ever done. It goes without saying that, like Rowe and Dunn, Mitchell feels my depression could easily have been caused by my relationship with my brother.

The last person I visited on my psychological journey was Peter Fonagy, Freud memorial professor of psychoanalysis at University College London, probably the leading developmental psychologist in Britain. He was less convinced that damage was inevitable. "Separation doesn't have to be a terrible thing. With one stressful experience most children will be resilient."

But Jeff had a number of stressful experiences – the mother returning depressed, the loss of his home, the sickly usurper in the house. Fonagy pointed out that Largactil was "very heavy – your mother was probably suicidal or not in touch with reality".

I ask if this experience was bound to cause damage to Jeff. "As I say, it depends on the context. Genetics are important, a child's natural capacity is important. The more stress there is on the child, the more negative outcomes – it can make it difficult to attach to anybody. It may make him want to harm his sibling. But none of it is inevitable. Two out of three people with early difficulties do perfectly well."

I am vaguely disappointed by Fonagy's optimism. I had three votes in my favour. His, the fourth, was more equivocal, but it instinctively felt more realistic. The truth is that, like the rest of my family, I'm inclined to believe you get over these things. All the psychological theories for behaviour are just clever stories, after all. They may be true or they may not. You tend to believe what suits you.

My brother and I are now close friends, and grow closer as the years pass. I confronted him with the studies. He was unimpressed. "What difference would it make now if they were right? What difference would it make if I admitted that I had caused you damage?" he said, calmly.

And after thinking about it long enough, I had to admit to myself – none whatsoever.

• Under the Same Stars by Tim Lott is published by Simon and Schuster, £16.99. To order a copy for £13.59, go to theguardian.com/bookshop or call 0330 333 6846