The nation's most prestigious body of scientists, the National Academy of Sciences, declared yesterday that the question of when human life begins is not a scientific one, thus inserting itself into the controversial abortion hearings now on Capitol Hill.

The academy singled out the definition in the anti-abortion bill sponsored by Sen. Jesse Helms (R-N.C.) and Rep. Harry Hyde (R-Ill.), saying that the bill's definition of when human life begins has no scientific validity. The academy, which is holding its annual meeting here, includes many of the nation's most distinguished biologists and physicists, including virtually all its Nobel Prize winners.

Congress is now holding hearings on the Hlems-Hyde bill, which is intended to redefine when human life begins, placing it at the moment of conception or shortly thereafter, and thus outlaw abortion by making it murder.

In other votes, academy members also came to the aid of their beleagured colleagues in the social and behavioral sciences, voting to protest proposed administration budget cuts targeted at those fields, leaving the natural sciences and engineering in relatively good shape.

"This was, in essence, the biologists and physicists going on the record to support their colleagues in the social and behavioral sciences," said academy spokesman Howard Lewis. "The recognize that if the social and behavioral sciences get their money put back, it will mean taking money out of other parts of the budget. They are saying that they are willing to have that done."

The academy in two separate resolutions challenged the budget reductions, which it said would cause "severe damage" to social and behaviorial science. "It's not the total reduction that is disturging," said academy President Philip Handler. "It is the nature of the individual actions and the manner of their doing. . . . If the government's science budget can be fashioned by such ideological behavior, which field of science will next suffer political attack?"

Handler said the administration is launched on a huge social experiment with its new economic program "but this vast experiment can only be appraised if those trained in such skills [as social science] . . . are afforded the means and given the opportunity so that, however it all turns out, we may learn by the experience."