On March 23, 2012, supporters from both sides of the abortion issue share the sidewalk next to the Planned Parenthood clinic in West Glenwood, Colo. (Kelley Cox/Glenwood Springs Post Independent via AP)

It’s clear that Congress has become more polarized in the last few decades — there are more conservatives and more liberals, and fewer moderates — but has the general public become more polarized? That’s a more difficult question, but we can use the General Social Survey, a large national survey that began in 1972 and has been conducted every year or two since then, to try to answer it.

And attitudes toward abortion are one critical measure of polarization.

The GSS regularly asks “whether or not it should be possible for a pregnant woman to obtain a legal abortion” in a number of different circumstances. Ever since the 1970s, overwhelming majorities have supported legal abortion “if there is a strong chance of serious defect in the baby,” “if the woman’s own health is seriously endangered by the pregnancy” or “if she became pregnant as a result of rape,” while less than half have supported legal abortion if “the woman wants it for any reason.” And yet if we look closely, we can see that public opinion has had some small but definite changes.

For one thing, fewer people support abortion in what are generally considered the most extreme or extenuating circumstances. Support for abortion if there is a chance of serious defect has fallen from about 85 percent in the 1970s to about 75 percent today. So has support for abortion if the pregnancy was a result of rape. Until the 1990s, more than 90 percent of Americans supported legal abortion if a woman’s own health was seriously endangered; in the last 10 years, the average has been about 87 percent.

And yet at the same time, more people support legal abortion for any reason, no matter why a woman wants it. About 35 percent held that view in the 1970s — but the number has risen to 45 percent in 2014.

While we can’t be sure exactly when views changed, the shifts seem to have been pretty steady, with gradual movement from the 1970s to now.

WEAKLIEM abortion

In short, over the past 40 years, public opinion about abortion has become more polarized. More people say it should be illegal under all or almost all circumstances; and more people say it should simply be legal without any conditions. The number of people who hold the middle view — that abortion should be allowed in some circumstances but not in others — is declining.

Why? What could cause a change like this?

People might be influenced by the positions of the parties. Democrats have united around a position of almost complete support for legal abortion. Republicans have taken an increasingly hard line against abortion; several of the current Republican presidential candidates say it should be legal only if the woman’s life is in danger.

Abortion is pretty consistently in the news; unlike in economics and foreign policy, there are few dramatic and unexpected events that could make people suddenly change their perspective. As a result, people may gradually be drawn toward the position of the party they trust, or pushed away from the position of the party they dislike.

At least on this one issue, it’s not just Congress that has become more polarized. It’s also the public.

David Weakliem is professor of sociology at the University of Connecticut. He blogs at Just the Social Facts, Ma’am.