Most of us will admit, however, that it is worth it. A great passion overwhelms us and grows for our children as they grow up. We simply take the emotional steps necessary to do this and never turn back. We begin to utter those words and we wait for the moment when they will say them back to us. The smiles and coos work for a while, but eventually we need a bigger return on our investment. We need to hear those words. I love you.

But this is a dubious event, with no prescript guiding our children to it, no law stating that a child must declare love for his or her mother or father or anyone else -- with or without conditions. What if Murphy's law ruled, and all the diapering, bathing, feeding and playing led to just a sort of mild appreciation?

We are all familiar with what happens when a child is deprived of maternal love (or love of any kind) during those crucial developmental years -- the possibility of physical debilitation, mental illness and even psychosis. But what of a mother who is deprived of a child's love? The bleak possibility leaves me feeling inept and afraid. Each day my son and I visit tiny traumatic experiences, unavoidable really, but I wonder if they make a lasting impression, deep enough to wipe away any growing feelings of affection.

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Love is obviously a dominant trait among human beings. As a biologist, I have wondered if there might not be a gene that controlled it, spread it throughout the human population and made it subservient to the principles of heredity. But this was wishful thinking on my part, less the hypothesis of a scientist than the optimism of an insecure mom.

Actually, the idea is not so far-fetched. The Human Genome Project is making great strides in helping to uncover genes for numerous social behaviors or, rather, aspects of human behavior that were once relegated to the realm of sociology. For example, we have laid claim to the discovery of genes for shyness, aggression, depression and homosexuality. (As an aside, I should mention that, as with most things, how genes are expressed is a far more complex process than a term such as "the gene for shyness" implies. But there is no space here for a lesson in genetics.)

Some 25 years ago, in his book "Sociobiology: The New Synthesis," Edward O. Wilson combined evolutionary biology and animal behavior to posit the historical reasons for the existence of various social behaviors in all organisms exhibiting them -- slime molds as well as humans. He used the tenets of population biology, ordered by natural selection and biological fitness, to look at societal comportment. As for love, he interpreted it as a part of self-sacrificing behaviors like altruism and prolonged child care, and, of course, as a component of sex. (He continued developing his unifying theory in "On Human Nature," published three years later, in 1978).

The Human Genome Project, in many regards, is the molecular equivalent of sociobiology and even, to some extent, social Darwinism. But because of its pop science flavor and purported medical benefits, it does not have the same negative buzz as either of its predecessors. Sociobiology and social Darwinism both were contentious issues and afflicted in their time. (Maybe because of statements such as this: "The pattern of parental care, being a biological trait like any other, is genetically programmed and varies from one species to the next," taken from Wilson's text.)

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