Mona Weissmark grew up without any cousins, aunts, uncles or grandparents, but she only understood why her family tree had been reduced to a stump when she was seven. It was 1961 and her mother was watching the televised trial of the Nazi, Adolf Eichmann. "That's him," her mother cried, "one of those bad German people. He killed my family." Then she explained that the number stamped on her arm was not their telephone number but the number she had received at the Auschwitz concentration camp. Her husband had survived Dachau and Langenstein-Zwieberge - all other members of both families had been murdered by the Nazis.

The grim revelation filled the little girl with a powerful feeling of injustice and indignation, and a deep-seated hatred of the Germans. But it also triggered a sense of curiosity about "the other side". When she saw Eichmann on television, sitting in an armoured-glass booth, "looking like a monster", she wondered: "Does he have children? Do they love him like I love my mother?"

It was from this sense of curiosity - academic and personal - that Weissmark later developed the idea of bringing the sons and daughters of Holocaust survivors face-to-face with the children of Nazis. "As a woman and a psychologist with a background in family therapy, I have always been concerned about relationships. I had long hypothesised that the children of victims and of victimisers have a lot in common," she says.

"The fact that my parents were survivors has affected my whole life. Was it the same for them? Did they feel guilty? Did they think like their parents? I wanted to know."

Weissmark, 49, an associate professor of psychology and founder of the Mansfield Institute for Social Justice at Roosevelt University in Chicago, sensed it was crucial that the two antagonistic groups should physically meet. She had the professional experience to conduct this unprecedented social experiment and, as a child of survivors, the moral authority to do so.

So it was that in the autumn of 1992, an extraordinary meeting took place at Harvard University. For the first time, the children of Holocaust survivors and Nazis sat down to talk over four days, discussing their painful, intertwined heritages. A subsequent meeting took place in Germany. The intention, says Weissmark, "was not to forget or forgive the past, but create a new future".

The sessions were strained at first. Both groups felt they were betraying their parents just by meeting the other side. Yet as the days unfolded, they realised that they shared the same overwhelming sense of guilt, anger and resentment, either because of what their parents did or because of what they had to endure. "They felt they had inherited a dark legacy submerged into their identities which consumed large parts of their lives.

"If left unresolved, the trauma of any atrocity inflicted on a group is then passed along to the next generation in the form of stories told about the parent's suffering, and those in turn lead to entrenched ethnic tension and group conflict. Unjust acts that have not been reconciled are stored in legacies as if packed in ice," Weissmark says. Now she has chronicled the moving, fraught, angry, but ultimately productive discussions between the two groups in her new book, Justice Matters: Legacies of the Holocaust and World War II.

The meetings didn't resolve every issue, but they showed that a rapprochement between second-generation victims and victimisers was possible. "There is a tremendous feeling of relief, a sense that I am leaving behind a lot of anger, a lot of resentment, a lot of fear," said a survivor's daughter at the end of the Harvard meeting.

Weissmark had observed this rebalancing process during the meetings. Years later, she had to journey through it herself. While she was writing the book, pieces of her family history slowly emerged, things she had buried because they didn't fit with her need to demonise all Germans. "I remembered conversations with my father when I was about 15. He would always start in hushed tones: 'There was a nice pastor who saved my life.'"

In April 1945, Weissmark later learned, her father Adolf and his friend, Rudolf Klepfisz, had fled Langenstein-Zwieberge, an offshoot of Buchenwald, and managed to reach the nearby village of Börnecke. Starved, and ill with typhus and dysentery, they had collapsed on the doorstep of Julius Seebasza, a German pastor. His wife and daughters, Renate and Ricarda, nursed them back to life. They stayed with the family for a few months until they could emigrate to the US. Ricarda, who probably contracted typhus from them, died later in the year.

It might seem surprising that Weissmark's father didn't make a bigger case of the Seebaszs' actions, but it is consistent with the survivor's psychology, she says. "Present in my parents' heads was the terrible loss, the murder and devastation - not those who helped him. When you have experienced such devastation, you cannot fit in this other category, you have an undifferentiated view of the other side: they're all bad."

Even if her father had told her more, Weissmark says she wouldn't have listened. "For years, I ignored and belittled what Seebasz did for my father, because I grew up with a deep hatred of all Germans and was not prepared to make an exception. In all these years, I never thought of the pastor, it only slowly emerged in the writing of my book itself. Even at that point, I never thought I would go and say thank you. I didn't even try to contact them. My husband did."

After months of hesitation and research, Weissmark finally managed to track down the pastor's surviving daughter, Renate, who is now 83 and a sister in the Convent of the Holy Name in Derby. When the two women talked on the phone for the first time in October, Weissmark says they had "the most amazing, intense, warm and exhausting conversation".

Sister Renate remembers Weissmark's father vividly. "I was 20, the same age as him. I had just come home from the front where I worked as a ground staff. We saw two people collapsed in front of our door. My mother called the doctor and he said: 'They are not human beings any longer, forget about them.' But my mother didn't listen," says sister Renate in her quiet, steady voice.

"Since then, I treat life as important and precious, having seen that two skeletons could regain their humanity after receiving not so much food, but acceptance and love."

Now the two women speak on the phone for an hour every Thursday. "Sometimes children have to finish what their parents couldn't. That's what I am doing with sister Renate."

Recently, Weissmark started talking with her daughter, Brittany, about her family's legacy. Brittany is now seven, the same age as her mother was when she learned about her past. "I realised that I started our family history with the positive, with Pastor Seebasz and Sister Renate," she says.

"Perhaps resentment against all Germans belonged to my generation, maybe gratitude towards some Germans can belong to my daughter's."

· Justice Matters: Legacies of the Holocaust and World War II by Mona Sue Weissmark will be published by OUP on June 3. To order a copy for £14.99 inc UK p&p call the Guardian Book Service on 0870 836 0875.