Children born from donated sperm have a great curiosity about their biological fathers and would like to contact them, but they are not likely to turn up on the doorstep and insist on being part of their life, a new study shows.

The findings of the study, published in the journal Human Reproduction, may reassure some would-be sperm or egg donors, who from next year will lose the anonymity they were guaranteed in the past.

Donor offspring born in the UK after April will have a right to know the identity of biological parents.

The study carried out among the offspring of sperm donors in the US is small but the first to give a picture of children's feelings. Researchers questioned young people from 29 households with an average age of 14 who had always known the truth about their parentage or were told it when they were very young.

They found that "with few exceptions, youths who have open-identity donors were comfortable with their origins". Knowing the truth "had either a positive impact or no impact on the relationship with their parents".

Almost half - 44% - said they felt having a donor did not affect their life.

More than 80% said they would like to contact the donor at some time.

They wanted to know what he looked like and whether they looked like him.

Most said they would like a picture.

"It appeared that while offspring were very curious and eager to learn more about their donor, they were also concerned about respecting his privacy," says the author, Joanna Scheib of the University of California, Davis and the Sperm Bank of California.