[This month, I'm serializing my 2003 Harvard Law Review article, The Mechanisms of the Slippery Slope.]
The discussion above has assumed that we know up front the preferences people have among positions 0, A, and B. But sometimes B might not even be considered at first, and the apparent choice might just be between 0 and A: for instance, returning to the enforcement need slippery slope example, shall marijuana be legal (0) or be subject to mild penalties (A)? Instead of the complicated mix of preferences I described in that post, we might at first see just two large groupings: a minority opposing the mild prohibition, and the majority supporting it. The risk that the mild law won't be complied with, which might lead to sterner measures, might not be much discussed.
Then, once the mild prohibition A proves ineffective, the hard-core war-on-drugs plan B will be brought up, and may succeed politically (if the preferences are as described in my earlier post). Some supporters of the mild penalty plan would regret their actions: they would rather stay with A, or even go back to 0, because they strongly oppose B; but their endorsement of A back when B wasn't even discussed now makes B possible. They might wish that they had thought earlier about the enforcement need slippery slope—but it would be too late, because there would now be a majority for going all the way to the newly proposed B.
This slippage is especially likely if A fails not just unexpectedly, but also because of changed circumstances. Say the United States is deciding whether to commit troops to a small peacekeeping mission in a foreign country (A). When this decision is being made, committing more troops to a broader military action (B) may not even be contemplated. But if the modest disorder that prompted the mission turns into a full-scale war, option A would no longer be feasible; we'd have a multi-peaked slippery slope, and A might slip to B even though B wouldn't have been authorized up front, without the initial step A.
And slippery slope risks might also be hidden—especially from average voters—by information asymmetry. Voters might not know exactly which step B would be proposed after step A is adopted. They might not know whether the results of step A would prove to be politically stable, or whether there are enough voters or legislators whose multi-peaked preferences would cause slippage to some broader result.
But voters might suspect that the politically savvy interest groups that are proposing A do know more about likely future proposals and likely voter preferences, and that those groups won't be satisfied with A but will push for something more. Sometimes A's advocates have explicitly said as much. Sometimes the proposal seems so unlikely to achieve its stated goals on its own that voters might reasonably conclude that it will surely be followed by other proposals. And sometimes voters might reasonably infer from the group's ideology that A isn't the only thing on the group's agenda.
What then should voters do, given their desire to make decisions without spending a lot of time and effort investigating the true magnitude of the slippery slope risk? One possible voter reaction is what might be called the ad hominem heuristic: if proposal A is being championed by a group that you know wants to go beyond A to a B that you dislike, you should oppose proposal A even if you mildly like it or have no strong opinion about it.
This heuristic seems similar to the ad hominem fallacy, in which a speaker asks listeners to reject certain arguments because the arguments are promoted by a group that the listeners dislike. We are properly cautioned to be wary of ad hominem arguments and to focus on the merits of the debate rather than the identities of the debaters.
But voters often lack the time and the knowledge base needed to evaluate proposals on their merits. Rationally ignorant voters need a simple heuristic that they can use when evaluating uncertain empirical matters, such as the risk that some behind-the-scenes mechanisms will cause proposal A to lead to result B. It is therefore rational for pro-choice voters, for instance, to reason that "If a pro-life advocacy group is for proposal A, then this increases my concern that A will lead to B, a broader abortion restriction, and persuades me to oppose A." {This may be just a special case of a broader ad hominem heuristic that arises even outside the slippery slope context—busy people who don't have the time to study some proposal deeply may rely on its supporters' and opponents' identities as a convenient, though highly imperfect, cue to what their own views on the subject should be.}
This heuristic can only be a presumption: if a voter sees that A is very appealing, or that the chances of A leading to some bad B seem especially low, then the presumption would be rebutted, and the voter should be willing to consider A on its own terms. But the presumption may make a difference in many cases—unless the voter sees some substantial benefit to A or some strong assurance that A won't lead to B, the very source of A's support can reasonably lead the voter to oppose it.
Unfortunately, even if the ad hominem heuristic is rational from each voter's perspective, it might have harmful social consequences. Making decisions based on hostility to various advocacy groups could worsen the tone of political debate by fostering a culture in which more time is spent demonizing a proposal's supporters than debating a proposal's merits.
Moreover, the ad hominem heuristic may exacerbate the slippery slope inefficiency—the social cost created when useful modest steps A, which on their own may be widely seen as socially valuable, are rejected because some voters fear that such steps A will lead to broader steps B. Nonetheless, voters may reasonably conclude that time and information constraints make the ad hominem heuristic a valuable tool, which they can't afford to abandon even if it lowers the tone of political debate.
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