Even three-year-old children of sperm donors, brought up by single mothers, ask about their biological fathers. Analysing the emotions of these families and the children showed that mixed feelings and unease exist among them.

Cambridge researchers compared information collected from 51 single-mother and 52 two-parent households which have children between 4 and 9 years, conceived with donated sperms.

Both the groups seemed to be "generally well-adjusted" and faced low levels of bullying in school, along with parallel contentment rates.

However, children brought up by single mothers explained that lacking fathers has led to "negative and mixed feelings". Just 4 percent of these sperm-donor children reported feeling positive about the state they were in. About 35 percent of them reported negative or mixed emotions.

The remaining women said that they felt "neutral" or "unsure" about the ambience. However, they admitted that the children begin to ask about their fathers from as young as three years.

One single mother of a five-year-old said that the first question asked to her was about his father. "Mummy, why don't I have a daddy?" he asked, from the back of his car. Hence, most mothers felt ambivalent about taking their decision to become pregnant through sperm donors.

Lead researcher Dr. Sophie Zadeh said: "Most of the mums would have preferred a traditional family set-up, and it's not surprising that some of the mothers reported that their children feel negative or have mixed feelings about the absence of a father, because they live in a world where a nuclear family is still largely the norm."

Still, the experts discover that sperm-donor children of even single mothers are "psychologically healthy," with 51 percent of them not too keen to change things. Even four- to nine-year-olds in single-parent families appear to be thriving and well.

These feelings can change when the sperm kids grow older, said Zadeh. When they touch puberty, they can carve out their own identities.

Some psychological challenges faced by the children were pointed out by Family Education Trust Director Norman Wells. He said that he felt some concern about their well-being: "This study confirms that many children produced without there being any relationship between their biological parents face significant psychological challenges."

The research is published in the Journal of Family Psychology.