"Why isn't Daddy here?" the 3-year-old asks.

"Daddy," says the mother carefully, "Daddy had to go live in another place called a jail because he did something bad."The child is quiet.

Then he asks, "If I mess my pants, will I have to go live in another house?"

"You'll have to do some talking to figure out that one," says Dr. Kyle D. Pruett, professor of psychiatry at the Yale Child Study Center and a "father advocate," author of "The Nurturing Father."

"At that age," Pruett told a group of child-development specialists, "they can already begin to understand the consequences of behavior but may make the assumption that the badness is somehow part of their own life as well."

Pruett and Dr. H. Joseph Horo-cek, psychiatry professor at the University of North Carolina and consultant at a North Carolina state prison, led a workshop on the effects of "Absent and Incarcerated Fathers" on their infants and young children at the annual conference of the National Center for Clinical Infant Programs. Fathers-in-jail is, in many ways, a metaphor for fathers-gone-to-war or fathers-in-hospitals. However, because of the punishment aspect and because of the good-bad implications, it can be vastly more complex. It is also vastly under-researched.

It is unclear what the absence of the father does to the developing infant. As Pruett put it, "The only statistics we have are for women. We have taken a good look at the literature and except for some good literature on incarcerated mothers, we were unencumbered in our presentation by a literature search. There is none."

Studies showing the need for maternal care have resulted in "small pockets of enlightened programs" in this country, in which nurseries have been set up in women's prisons, for example.

But nothing for men. Yet of the 750,000 people in American prisons, 90 percent are adult males. About 60 percent are married before they go to jail. The average stay for women is about 18 months; for men, between two and three years.

The chances that "daddy" will come back into his child's life, then, are very good.

There are, said Pruett, about 300,000 children of jailed fathers in this country, with an estimated 80,000 under the age of 2.

And although the requisite studies have yet to be done, both Pruett and Horocek, who has worked mostly with the jailed fathers themselves, are prepared to extrapolate from their own clinical observations and studies that have been done in related areas.

Said Pruett, "There are some differences between maternal and paternal interaction with infants and young children that make father absence more critical than we may have thought."

Citing studies conducted by pediatrician Dr. T. Berry Brazelton and other child-development researchers, Pruett noted that new information about the "differences in the way fathers and mothers interact with their infants and young children" may shed new light on the studies done on paternal absence during World War II, mainly in England, by Dr. Anna Freud, among others.

"In the first place," said Pruett, "right from the outset, fathers don't mother and mothers don't father. The innate differences in handling styles are quite discrete.

"There is some overlap, but there are also some separate, easily definable differences between mother and father interaction with children.

"First, fathers pick their babies up differently than mothers do. Research shows us that when a mother tends to approach her infant she will often make visual and verbal contact with the infant. Typically, she will bring her hands around down by the side of baby and will bring the baby up to the upper part of her body, cuddling it in the crook of her neck so that the baby will be enfolded in the mother's psychological and physiological space."

Moreover, said Pruett, "most mothers tend to do it the same way. It is predictable. They even do it with other babies than their own."

"If there is anything predictable about the way fathers do it, it's that it is unpredictable. A father will tend to approach a baby differently even at different times on the same day. He may surprise the baby, with no verbal or eye contact before he picks the child up, and then once he does pick the baby up he may not bring it up to his body for some time; he may turn the baby over in his hands; he may hold the baby at arm's length; he may toss the baby into the air and by the time he does bring the baby to his body, the baby is held in apposition to the father's body in a way that permits the infant more access to the outside world and the environment.

"One of the typical ways fathers transport their children is in the so-called football position, where the baby is held in the crook of his arm and the baby and the father will then sort of encounter the world en face, together. Instead of being folded into the internal space, as with the mother, the baby joins the father's body."

Pruett noted that videotapes made in Brazelton's Harvard clinic-laboratory show that 6-week-old infants can distinguish between the approaching mother and the approaching father and seem to be able to tell what is going to happen when contact is finally made.

When a mother approaches a baby, the baby's heartbeat and breathing seem to settle down and regularize. The baby closes his eyes halfway and "sort of scrunches around as if to say, `here comes something real good.' "

With the father, it is quite different. Heart rate goes up, breathing quickens and eyes open as if to say, `Here comes party time.' "

Studies of infant-father interaction have suggested that the more involved the father, the higher the 6-month-olds scored on mental and motor development tests. One psychologist cited in Pruett's book found that infants at about two months were more socially responsive and better able to handle stresses when the father was involved even in routine care-giver tasks such as bathing, changing diapers, feeding and dressing.

And as the baby develops, the father continues to be a presence that suggests adventure, socialization, connection to the outside world, someone who helps the developing child learn to interact normally as its symbiosis with the mother gradually weakens. Pruett calls these "enriching and slightly different interactive opportunities (from the mother)," which are absent when the father is absent, causing, he said, "a deficiency in the infant's early life of the organization of these early enriching experiences."

Imprisonment of a father becomes different from other absences when the child reaches toddler age. The question of when and how to tell takes on unique importance, as does the decision as to whether or not to maintain a tie with the father while he is in prison.

But maintaining a relationship during the imprisonment, said both Pruett and Horocek, is not always best for the child. A child may need to replace the father with a male role model who is available all the time.

"Remember," Pruett cautioned, "that the child needs to sustain relationships he needs on a day-to-day basis. Children cannot wait. A 2-year-old cannot put off daily needs for a few years for a relationship with father until he's out of prison."

On the other hand, when a child asks about his father, it is not constructive to answer, "I want to tell you (how bad) your dad was, and I really hate him and never want to see him again. You want to know? You want to know? That's why he's gone." A not uncommon response.

The trouble is, said Horocek, "society in general is not sympathetic to fathers in jail. There is less consideration for the impact of fathering on a developing child and more a feeling for cutting him off from the child because he is too evil."

Both specialists agreed that the decision to encourage or discourage a continued child-father relationship while the father is in prison is not an automatic one, but, said Pruett, "it is important for someone to think about it."