Can you outsmart the slippery slope fallacy? - Elizabeth Cox

On the plains of the Serengeti, a dung beetle rolls his perfectly sculpted ball of dung away from competitors. In this Canadian river, a beaver rushes to reinforce her dam as it threatens to burst. As the snowball thunders down the mountainside, gaining momentum, the arctic foxes run for cover— I can't stand these nature programs. Always the same story, and not a rational actor in sight.

What else is on? It’s April 1954, and Vietnamese nationalists are on the verge of victory against French forces fighting for control of Vietnam. Their victory could lead to an independent Vietnam under communist leader Ho Chi Minh. The United States President, Dwight D. Eisenhower, is holding a press conference to comment on these developments.

Well, if you really must. Eisenhower claims that by virtue of what he calls the “falling domino principle,” communist control of Vietnam would be the “beginning of a disintegration” that would be certain to cause “incalculable loss.” The beetles and beavers may be beyond my reach, but surely here's someone I can reason with.

Now, Mr. President, let’s take a deep breath, shall we? It’s a big leap—or, one might say, a long slide—from communist governance of Vietnam to the global spread of authoritarian communist regimes. It’s as if we were to say you were clothed, now you’re in your underwear, so soon everyone in the world will be completely naked. Don’t worry, I may have that power, but I promise not to use it.

Now, as I was saying, this kind of argument, where one step, let’s call it A, kicks off a string of events that inevitably culminates in an extreme scenario, let’s call it Z, is known as a slippery slope. Many such arguments focus on catastrophe, but the slope to an extreme positive outcome can be just as slippery. The trouble with this kind of argument is that, in presenting Z as the inevitable outcome of A, it almost always overstates the likelihood that Z will happen if A happens.

Why? Allow me to trouble you with some math. Let’s assume for the sake of argument that, taken individually, each step between A and Z is independent from the others and very likely—99%. So the probability that A causes B, that B causes C, that C causes D, and so on, is each 99%. Even so, each additional step adds an opportunity to alter the outcome, and A is only 78% likely to lead to Z—far from an inevitability.

If there’s a 95% likelihood of each step, the chance that A leads to Z plummets to about 28%. If there’s a 90% likelihood at each step—still very likely by most standards—the chance that A leads to Z is only 7%. And if 24 of the 25 steps between A and Z are 99% likely, and one is 50% likely, the chance that A leads to Z goes down from 78% to 39%.

Back to your situation. I won’t deny you have reason to be concerned. You’re warily watching as powerful authoritarian communist regimes in the Soviet Union and China try to spread their form of governance. But let’s take a look at the chain of events you suggest: You say that the countries surrounding Vietnam would all soon fall under communist rule; that this would result in a loss of essential trade with these countries for others; that with no non-communist nations left to trade with, Japan would be pressured towards communism and that this, in turn, would threaten Australia and New Zealand. Your ultimate fear, if I may presume, is that this will in turn threaten the United States.

Is this a possibility? Sure. Where I take issue is with your comparison to dominos. These complex real-world events are not, in fact, like dominoes, where when the first one falls, it becomes a certainty that the last will fall. For any one of these events, a number of possible outcomes could result, each affecting the other events in different ways. The possibilities are not a chain, they’re a web.

It’s 1975, and after 20 years of conflict, and several million lives lost, North Vietnamese forces have taken control of the capital of South Vietnam. The war is over, and all of Vietnam is under communist control. Communist regimes have come to power in neighboring Laos and Cambodia, where the regime will be responsible for the deaths of an estimated quarter of all Cambodians.

Wait, there's more. That first step you were trying to avoid happened, but the end result you predicted did not. As for the steps between, a few happened; many did not. Decades afterward, your fellow humans are still debating why events unfolded the way they did.

And this is the trouble with slippery slope arguments. They focus exclusively on extreme outcomes, assigning those outcomes a degree of certainty or inevitability that rarely corresponds to reality. They divert attention from other, more likely possibilities, foreclosing discussions that might be more productive. And that’s when they’re made in good faith. Slippery slope arguments can also be intentionally structured to take advantage of people’s fears—whatever your position on an issue, it’s easy to come up with an extreme outcome that suits your aims. Best to avoid them entirely, eh?