Many who oppose sex education for the very young fear that it will corrupt their minds and take away their innocence. But a new study has revealed that sex is by no means an alien concept to young children - and they are keen to know more.
Sex education researchers found that some young children were already able to pose questions about same-sex parents and could provide alternative words for their genitalia.
But taboos around sex could make parents close down more detailed conversations about how babies are conceived, the study found.
The findings come as debate rumbles on about the correct age to teach sex education, and whether to teach it all. In the UK, it is currently only statutory from secondary school, and attempts to teach it in early primary school have met with opposition from some parents and religious groups.
The researchers asked parents to read one of four picture books to their four- and five-year-old offspring. These books ranged from a simple guide to pregnancy and birth to a comprehensive look at intercourse, same-sex parents and masturbation.
Inevitably, the discussions between parent and child were subject to misunderstanding, with one child informing her mother: "Dads can have babies. They take it out of your neck."
But the researchers, from the University of Michigan, found that children were capable of bringing their own experiences to an understanding of sexuality. One five-year-old boy used the knowledge that he had a classmate with two mothers to extend the conversation beyond the limits imposed by his own mother.
Told that it takes a man and a woman to make a baby, the boy replied: "My old classmate, Sean.he was just born by two moms. How did that happen?"
"The boy's question added a layer of complexity to his knowledge," the academics write in a paper published in the latest edition of the journal Sex Education.
Similarly, children would occasionally counter the information they were being given by their parents with contradictory information of their own. For example, parents tended to prefer anatomical terms for genitalia, or a euphemistic "privates". One child, however, pointed at the picture of a boy's genitals and said, "Weiner!"
Children's minds moved quickly, jumping from sexuality to religion, philosophy and science. They asked questions about where the world came from, what cells are and what blood is. "Who's the first person?" one child asked. "Who were the first people born? Who was the first ones alive?"
And they were persistent in pursuing information that they felt was pertinent to the discussion. Talking to his four-year-old son about the umbilical cord, one father made an off-the-cuff comment about the boy's mother not drinking coffee, beer or wine while she was pregnant.
"Did you drink coffee? Why?" the boy asked, pointing out that the baby came from the father as well as from the mother. Then, keen to understand better which part of him came from his father, the boy continued: "Would it be my head? Was it my head or my toes?"
Not all persistence was rewarded with answers, however. Parents were particularly reluctant to answer questions about sex itself and would shut them down quickly. One five-year-old boy, told that both fathers and mothers have parts in their bodies that make a baby, asked for more information. "It's kinda for when you're a little older," his mother said.
"This foreclosure is likely to let the child know.that there is something taboo about how the baby gets `inside'," the researchers write.
"Children are actively engaged in shaping what they come to learn and know about sexuality," they continue. "The outcomes of children's knowledge.are not solely a product of what [adults] decide to tell them."
Reacting to the findings, Jules Hillier of UK sexual health charity Brook said that she would like children to be given sex and relationships education from the early years of primary school. "We want children to learn about healthy relationships, to help prepare them for what the future's going to hold," she said.