Southern trees bear a strange fruit,

Blood on the leaves and blood at the root,

Black body swinging in the Southern breeze,

Strange fruit hanging from the poplar trees.

Pastoral scene of the gallant South,

The bulging eyes and the twisted mouth,

Scent of magnolia sweet and fresh,

And the sudden smell of burning flesh!

Here is a fruit for the crows to pluck,

For the rain to gather, for the wind to suck,

For the sun to rot, for a tree to drop,

Here is a strange and bitter crop.

The germ of the song was in a poem written by Lewis Allen. When he showed me that poem, I dug it right off. It seemed to spell out all the things that had killed Pop (Holliday's father had died of pneumonia in 1937 after several segregated southern hospitals refused to treat him). Allen, too, had heard how Pop died and of course was interested in my singing. He suggested that Sonny White, who had been my accompanist, and I turn it into music. So the three of us got together and did the job in about three weeks

I had always assumed that Billie Holiday composed the music and lyrics to "Strange Fruit". She did not. The song began life as a poem written by Abel Meeropol, a schoolteacher who was living in the Bronx and teaching English at the De Witt Clinton High School, where his students would have included the Academy award-winning screenwriter Paddy Chayefsky, the playwright Neil Simon, and the novelist and essayist James Baldwin. Meeropol was a trade union activist and a closet member of the Communist Party; his poem was first published in January 1937 as "Bitter Fruit", in a union magazine called the New York School Teacher. In common with many Jewish people in the US during this period, Meeropol was worried (with reason) about anti-semitism and chose to publish his poem under the pseudonym "Lewis Allan", the first names of his two stillborn children.

Even more than half a century on, it's hard to hear this story without being affected by its magnitude. As Robert Meeropol describes what happened on that evening 56 years ago, I have tears in my eyes. When Meeropol describes how, earlier that same day, his brother began moaning, "That's it then! Goodbye, goodbye"; when the news flashed on to the television that the executions were going ahead that night; and when he describes seeing the press reports counting down his parents' final days, I can hardly bear to listen.

Meeropol (whose name was later changed to that of the couple who adopted him) is used to journalists getting emotional on him. "It's different for you," he says understandingly, "I've lived with this all my life; I'm used to it." But how does anyone get used to the fact that their parents have been put to death by their country; how does anyone pick up the pieces of a childhood left that broken? What is most extraordinary about Meeropol, in fact, is how entirely ordinary he seems today. We meet in Berlin, where he is currently on a book and campaigning tour. Now 62, bespectacled and balding, he is every inch the liberal east-coast lawyer and grandfather he has become. Yet, as he's the first to point out, his life is permeated by the story of the parents he knew for such a short space of time: their legacy has taken up much of his life, certainly much of his last 30 years, and fighting against the death penalty, and being an advocate for children who suffer as he did because of their parents' politics, is now his full-time occupation...

Doesn't Meeropol ever feel, though, that the choice Ethel and Julius made was fundamentally selfish: that their most important role was as parents? "Absolutely not," he says. "The world was very different then: capitalism and communism were engaged in a globe-spanning battle to determine the world's fate. Lots of people chose sides in this life-and-death battle. Also, my mother didn't actively participate in what went on - maybe that was a conscious effort to ensure that at least one parent would be around to raise the children if my father was caught."

But even when they were arrested - Julius was taken first, then Ethel - there seems little doubt that they could have acted to save themselves. Wouldn't that have been better for their children? Again, Meeropol thinks not. "Neither of my parents had a choice whereby they could come forward and say, 'OK, I admit I've done this, now how can I save my life?' What the government wanted them to do - and remember this was the McCarthy era - was become puppets, to dance to their tune and to provide a list of others who would then be put in exactly the position they were in. They would have had to renounce all that they believed in. To save themselves, they'd have had to betray others and that was too high a price to pay."

But all this went way over the heads of the two small boys who suddenly found themselves without a mother and father, shunted from home to home while the sand ran through the timer counting down the final months and weeks of the Rosenbergs' lives. It's clear from everything he says that the events of that desperate time were almost unfathomable to him; it's clear, too, that he'd have given anything for an ordinary home and an ordinary family. He remembers, for example, seeing his cousins with their parents and thinking, why can't we be like that? But, interestingly, the adult Meeropol believes that, while the little boy he once was suffered for his parents' stubbornness in the face of death, the adult self he became has gained enormously from it. He is immensely proud of them, even grateful: he says he hopes that, in their shoes, he would have made the same decision they did - the decision not to betray their friends.

But more than that, what the Rosenbergs bequeathed to their younger son was something every life needs. They left him a purpose. Campaigning against the death penalty and working for his fund have given his life a structure and a cause: their decision half a century ago is continuing to shape his life.

Pull him back to his stories of the personal encounters he remembers with his parents, and it's clear, too, that he knows he was a much-loved little boy. The time Ethel and Julius had with him might have been short (he was three when they were taken away to prison), but they made it count with their love and concern. What is more - and this, too, is almost unbearably poignant - it's clear that they tried to parent him as best they could from their prison cells. There were letters - lots of them - all unfailingly upbeat and cheerful; there were visits...

The Meeropols, who were not friends of the Rosenbergs but were members of the American Communist Party, came into the boys' lives after a period of constant upheaval. From the time of their parents' arrests, and even after the execution, they were passed from one home to another - first one grandmother looked after them, then another, then friends. For a brief spell, they were even sent to a shelter.

It seems hard for us to understand, but the paranoia of the McCarthy era was such that many people - even family members - were terrified of being connected with the Rosenberg children, and many people who might have cared for them were too afraid to do so. After he and his wife had adopted the boys, says Meeropol, Abel didn't get any work as a writer throughout most of the 1950s. "I can't say he was blacklisted, but it definitely looks as though he was at least greylisted," he says.

His debt to Abel and Anne is profound: he feels he is at least as much a product of their upbringing as of that of Julius and Ethel. "They were childless, and like our birth parents they were people who believed in standing up for what they believed in," he says. "They were more artistically inclined than my parents [Abel wrote the anti-racism song Strange Fruit, sung most famously by Billie Holiday]."

It was, against the odds, a happy childhood, punctuated with visits to summer camp, music and fun. Very quickly, Robert began to call his new parents mommy and daddy; today, he says he feels he had not two but four parents in his life. "I'm the sort of person who finds the upside in life," he says. And having four parents was, he believes, a blessing.

Another blessing was Michael. In his book, Meeropol describes Michael as "the one constant presence ... in my life. Our four-year age difference diminished our sibling rivalry. We always slept in the same room." Before the Meeropols, Michael was "the only person I felt 100% safe with". To this day, the brothers are extremely close.

Having lost his parents, says Meeropol, family became paramount for both brothers: "Both of us married young, and both of us are still married to the person we married all those years ago. Creating a family, and maintaining it, has been central to both of us." Meeropol has two daughters, now in their 30s; the younger has a one-year-old called Josie. If there is anything that resonates down the years, he says, it is that he often finds himself thinking: if I was taken away, what would my family have to remember me by? What would my little granddaughter know of her grandfather if suddenly he was removed from her life?

If having the Rosenbergs as parents has given their sons a strong sense of family, it has also given them profound insight into what happens when a family is torn apart. Because one of the most remarkable aspects of the trial in 1952 was that it was Ethel's own brother, David Greenglass, who provided the testimony that sent the couple to their deaths.

Greenglass had been an army machinist at the plant where the atomic bomb was being developed, and was recruited by Julius as a spy; to save himself and his wife, Meeropol believes, he betrayed his sister and her husband. Unsurprisingly, this is a family split that never has been, and never can be, mended. "I have never had any connection with David Greenglass or the Greenglass family," says Meeropol. "I saw him interviewed on television once and the thing I noticed was how he denied responsibility for everything. Nothing was his fault - it was all someone else's fault." He pauses. "In some ways," he says, "I've defined myself, all my life, as someone who is not David Greenglass."

The fallout for his uncle and his family (there are two cousins, and now there are Greenglass grandchildren too) has been, in fact, a testament to what would have happened to the Rosenbergs if they had switched sides. "The Greenglasses had to have new names, they have had to live their lives in secrecy, they have lived in fear.

"What my parents gave me and Michael, though, was a life in which we have never had to hide, a life in which we can stand up and be ourselves and do the things we believe in." He pauses. "In a way," he says, "the best revenge is simply living a good life. And that's what I believe I'm doing."