There is a button inside most of us labeled Family Responsibility. Press it and you send a modest shock wave through the body, even the body politic, until it hits our nerve center of guilt.

The Reagan administration knows its way 'round these circuits well. Last week it leaned long and hard on this button. It said that states can now require adult children to help pay the nursing-home bill for their parents on Medicaid. It's called the Family Responsibility requirement.

This isn't the first time they've tried to cut costs and foster ideology at the same time. There is a general belief that we can and should take care of our own: defederalize into the arms of families.

The belief in a moral obligation to family is shared by most of us. We are vulnerable to charges of selfishness. We judge harshly the fathers who desert their children onto AFDC. We cringe at the notion that elderly parents of comfortable adult children may be on government poverty programs.

Yet there is something treacherous in trying to force parental support onto the list of legal family obligations.

The ethical bargain--parents raise children, and children care for parents in their old age--has never been pure. Since 1597, English law has tried to shore up moral responsibility to care for poor relatives.

In 19th-century America--when the economic family unit was disintegrating --family-responsibility laws proliferated. By the 1930s, 35 states had such laws, and all of them directed at the families of the poor.

As Michael Grossberg, a family-law historian at Case Western Reserve University suggests, "There was generally the attempt to use the law to get poor families to live up to the expectations of nonpoor families."

But in fact the laws were, as law professor Carol Bruch of the University of California at Davis says, "written in invisible ink." They were rarely implemented.

There was and is a prejudice in the United States toward supporting children over parents. Our 19th-century judges were reluctant to burden adult children in a way that would jeopardize the next generation. We hear this reflected among today's elderly who "do not want to be a burden."

Since the 1930s, we have built a system of pensions: Social Security, Medicare and even Medicaid to shore up and share our burdens. The elderly have voted again and again for the "independence" of government programs over dependence on their children.

Now, when the Medicaid cost of nursing-home care is between $10,800 and $12,000 a year, when the average age of a nursing-home patient is nearly 78, do we want to enforce a parental-support requirement on a 55-year-old son or daughter? Do we want to jeopardize the security of two older generations?

I touch this subject gingerly because I share a strong sense of moral obligation, family responsibility. I am convinced that most of us worry about and plan for and, if necessary, want to help our parents in old age.

I resent deeply the inherent, and unsupported, accusation that there are thousands of adult children living in luxury while their parents languish in nursing homes on Medicaid. They are as rare as welfare mothers in Cadillacs.

Still, there is a vast difference between a moral obligation and a legal obligation. Would the state act as family collection agency? Would it determine who can and can't help their parents? Would it then require the elderly parents to list all of their children's assets? Would the government assess and distribute the costs among sisters and brothers? What about grandchildren? Would this really foster family unity?

Finally, I find it pernicious that in a time when medical costs are rising outrageously, when the assets and savings of many elderly are devoured by these costs until they qualify for Medicaid, we are offered one solution: pass the bill to the children.

I'm glad that most of us have a button marked Family Responsibility. But where's the button in this administration that reads Social Responsibility?