Q: Your response to the mother who wanted only one child was helpful to me.

Many well-intentioned voices--friends, family, "experts" and even one's own guilt and insecurity--influence a parent's thinking. Like a Greek chorus, these voices hound you until you can't figure out what's best.

After months of listening, I've finally sorted out my own reasons--emotional and intellectual--for wanting my family to go on as three--my husband, my son and myself.

First, we have a nice dynamic. We're a comfortable, congenial unit; we're "pals." Another child would inevitably change that balance for months and maybe years. I'd be "coping with kids"--hearing what they need to keep quiet--instead of listening to them and appreciating them.

This concentration on one child mirrors the kind of people my husband and I are. We don't diffuse ourselves. We have a small set of friends; our interests are confined and intense. Our make-up, both individually and collectively, suits us to one child.

And then there is the very large complication of my work. Being away from my son all day, I want to give him my full attention during the evening. I don't dilute that attention with social, charitable or any other kinds of activities. To dilute it with another child is an overwhelming prospect.

There is also a strong tug to see a certain safety in numbers but I can't act on this logic. You can't ease the loss of one of your treasures by having some "spares."

For these reasons I choose to have one child. I feel comfortable with this decision, but it is in no way a "doctrine" I am trying to spread. I'd prefer that those well-meaning voices, carping about the "poor only-child syndrome," would stop trying to impress their doctrines on me.

A: From what you've written, you and your husband are much of a mind, but there are parents--perhaps not too well suited in the first place--who have different ideas of the perfect family. One aches to embrace an army while the other has a lap just big enough for one or two. It's here that couples often run into serious problems.

If you both agree on your analysis of your needs and the needs of the good family you've built, you'll be able to ignore all those other voices like so much crackle on the radio.

Your self-analysis is what every couple needs periodically, and especially before they decide to have another child. All parents have a level at which they function best, but they often discover it about one baby too late. It's the Peter Principle of parenthood. If only there were a yardstick or an aptitude test that could tell a parent when enough was really enough.

We've all seen families that grew in a harum-scarum way, only to have the enormity of that decision shatter the finances--or the marriage--as the first child reached a critical passage like 8 or 13 or 15.

To some couples, who want everything quite correct, even one child is devastating. Another family, like yours, is happiest with one, and still another can stretch their time and attention with little trouble.

There's a problem with stopping too soon as well as too late.

A couple may plan their life style with great care but forget to take their expansive nature into consideration and so devote too much energy to too few children--a blanket effect that can smother a child.

Of all the reasons to have more children, the safety-in-numbers factor is the most foolish. As you point out, a child is irreplaceable. Even an identical twin can't take the other's place.

And of all the reasons for having more than one child, the joy--and the surprise--of watching each one unfold, and so differently, is surely the greatest--and the most lasting. Worth Noting

The Laboratory of Behavioral Medicine at Children's Hospital has extra openings for participants in its study of learning-disabled children without neurological problems. For further information, call Mark Reader, 745-3247.