It is tempting to believe that a political party will act in accordance with its self-interest, which is to win elections. Democrats may be wrong, and Republicans may be wrong, but in the end, neither party will act in self-destructive ways.

For this reason, many people argue that in the House and Senate, Republicans will ultimately support immigration-reform legislation, and that they will enter into some agreement to increase the debt limit and avoid a government shutdown.

The predictions might turn out to be right, but don’t be too sure. The central argument is rooted in a logical fallacy, explored by Aristotle, called “the fallacy of division.” The fallacy occurs when people believe that if the whole has some characteristic, its parts will have that characteristic as well. Because the fallacy of division is important, and because it has so many implications, let’s explore it in a little detail.

Here are three examples. 1. A motorboat can move on water. A motor is part of a motorboat. Therefore a motor can move on water. 2. Citizens of France are thin. Jacques is a citizen of France. Therefore Jacques is thin. 3. The Baltimore Orioles hit a lot of home runs. Nate McLouth is a Baltimore Oriole. Therefore McLouth hits a lot of home runs.

When people commit the fallacy of division, they fail to appreciate the fact that a whole may have characteristics or abilities that its parts lack. A boat can do things its parts cannot do. The fallacy may also rest on a failure to see that a group typically has characteristics that some group members lack. A sports team may have identifiable characteristics, but some or many of its players may lack those characteristics.

Now turn to politics. Let us stipulate that it is in the self-interest of the Republican Party to support immigration-reform legislation. Even if this is so, it may not be in the self-interest of particular Republicans to support such legislation. Indeed, it may not be in the self-interest of most Republicans to support immigration-reform legislation.

The difference between the group’s interest and the members’ interest may occur for disparate reasons. But imagine that to be re-elected, many of the party’s legislators will need the “cover” of a strong Republican vote in favor of the legislation. If individual members risk their political future if they vote for the measure, they will vote against it, unless most or many of the party’s members can make a binding commitment to vote in its favor. That commitment might be exceptionally hard to obtain.

For a party’s members, some cases are even more difficult. Suppose that many voters will be unhappy if the Republicans block immigration reform, thus weakening the appeal of the party “brand,” reducing the number of self-identified Republicans and endangering the party’s prospects for capturing the White House in 2016. From the party’s standpoint, these consequences would be pretty awful. But from the standpoint of some or many individual members of Congress, they might not come close to tipping the balance.

Suppose that a member’s constituents are very conservative, that there is a real possibility of a primary challenge from the right, and that a Democrat can’t possibly win the seat. Under these conditions, a self-interested Republican might well end up voting in a way that will hurt the party.

In the current House, a large number of Republicans appear to think in this way, and some Senate Republicans do as well. Of course, legislators’ personal convictions may well matter, and lead them to vote for legislation they favor, but the role of electoral self-interest can’t be discounted.

The same dynamic affects Democrats. Because of electoral pressures, individual Democrats may support environmental, entitlement and civil-rights legislation that is to the left of the country’s position and harmful to their party’s electoral prospects.

None of this means Congress will refuse to enact immigration reform or pass legislation to prevent a government shutdown and to raise the debt limit. At least on fiscal issues, Republicans and Democrats have been able to find a path forward.

But let’s not forget Aristotle. When contested legislation passes, it isn’t because its enactment is in the interest of the Republican Party or the Democratic Party as such. It’s usually because it’s in the interest of the particular Republicans, and the particular Democrats, who end up for voting for it.

(Cass R. Sunstein, the Robert Walmsley University professor at Harvard Law School, is a Bloomberg View columnist. He is the former administrator of the White House Office of Information and Regulatory Affairs, the co-author of Nudge and author of Simpler: The Future of Government.)

Photo: Giovanni Dall’Orto via Wikimedia Commons