In the specific case of abortion, the matter is particularly easy in that no woman wants a late abortion. Once abortion was made legal, the age of the aborted fetus went down. The slope slipped in the other direction. If we legalize RU-486 and other similar new drugs, the age will fall to one week or less and start approaching zero. The slippery slope will slide in the other direction. The only reason we have late abortions is because we make early abortion difficult.