Central to almost every high school policy debate round is the concept of causality: one event is said to cause a second event, either good or bad. Debates are laden with the language of causality: “X is key to Y” is the most popular phrasing of taglines, as in “deficit spending is key to the economy” or “military readiness is key to hegemony”. But what does it mean for one thing to cause another? Philosophers have been discussing this very question for millenia and there is no easy answer, but the concept of necessary and sufficient conditions is one way to make sense out of claims of causality.

What is the difference between a necessary condition and a sufficient condition? And how can debaters use these concepts to improve their debating?

R. Mark Sirkin, an Associate Professor of Political Science at Wright State University, provides a clear introduction to these concepts in his 2006 book Statistics for the Social Sciences:

We should be aware of this distinction between necessary and sufficient. A necessary condition is a condition that must be present in order for some outcome (in this case, rain) to occur. Its presence, however, does not guarantee that the outcome will occur. By comparison, if a sufficient condition exists, the predicted outcome will definitely take place. For example, one could argue that poverty is a cause of communist revolutions. Indeed, the presence of poverty motivated Marx, Lenin, and Mao in their writings and strategies, and there was great poverty in prerevolutionary Russia and China. Yet, many impoverished nations have not undergone Marxist revolutions. Why a revolution in Cuba but not in Haiti? Perhaps poverty is necessary but not sufficient for such a revolution. Then, in addition to poverty, one or more other factors may be needed for a revolution, such as a perception of inequality, unmet rising expectations of an end to poverty, or an organized revolutionary movement. If the presence of poverty alone always led to leftist revolution, then it would be both necessary and sufficient. It is also possible that any of several conditions, when accompanying poverty, can cause revolution; for example, either poverty plus a perception of inequality or poverty plus a charismatic revolutionary leader is sufficient to bring about revolution. When we study hypotheses containing more than two variables, we take the necessary versus sufficient aspect of relationships into particular consideration. (p. 15)

When evaluating claims of causality, we can separate potential causes into three categories:

1. Conditions that are necessary (but not sufficient) for the outcome to occur.
2. Conditions that are sufficient for the outcome to occur.
3. Conditions that are necessary and sufficient for the outcome to occur.

For example, consider the causes of the American Civil War. What conditions were necessary for the war to break out, but not by themselves sufficient? What conditions, on the other hand, were sufficient to cause the war?

Robert Chadwell Williams, the Vail Professor of History and Dean of the Faculty Emeritus at Davidson College, uses this hypothetical to explain the differences between necessary and sufficient conditions in his 2003 book The Historian’s Toolbox: A Student’s Guide to the Theory and Craft of History:

For example, historians continually argue about, debate, and revise our understanding of the causes of the American Civil War. They usually distinguish between causes that were necessary and those that were sufficient to cause a civil war. Necessary causes include sectional differences between North and South, the existence of slavery as an institution, the moral and political critique of slavery, and the dispute over the tariff. Given the existence of these necessary causes, then other causes were in the end sufficient to lead to a civil war—the Kansas-Nebraska Act of 1854, the election of Abraham Lincoln as president in 1860, or the federal decision to resupply Fort Sumter in Charleston Harbor in 1861, and the southern firing on the fort. Without the necessary causes, the sufficient causes would probably not have been sufficient. With them, they produced war and endless suffering—and monumental changes—in both North and South. (See the longer example of causation and the Civil War in the toolbox section of this book.) Historians rarely make their causal models explicit. But they often assume them. [end page 17]

Causation is like an explosion. Necessary causes are like dynamite, plutonium, or hydrogen—that is, the fuel. Sufficient causes are like the fuse, match, implosion lenses, or atomic trigger—that is, the ignition device. Ignition causes explosion—but only because the fuel is present. (p. 17-18)

So how can debaters make use of these concepts in contest rounds?

First, identifying whether a potential cause is necessary or sufficient can help debaters substantially reduce the probability of their opponents’ impacts. If a link (or internal link) is a necessary but not sufficient condition for the impact to occur, debaters should argue that the impact is unlikely regardless of the link because a sufficient condition does not exist.

For example, an affirmative team might argue that a further decline in the global economy will result in outbreaks of violent conflict. In response, the negative can argue that economic decline is a necessary but not sufficient cause of global wars. While economic decline might need to occur in order for conflicts to break out, other conditions might also be required. So long as the United States maintains a strong nuclear deterrent and there is a robust system of global free trade, for instance, perhaps economic decline alone is not sufficient to cause war.

Niall Ferguson, the Laurence A. Tisch Professor of History at Harvard University, advanced an argument along these lines in a 2006 article in Foreign Affairs that many debaters cite to answer the common “economic decline causes war” impact:

Nor can economic crises explain the bloodshed. What may be the most familiar causal chain in modern historiography links the Great Depression to the rise of fascism and the outbreak of World War II. But that simple story leaves too much out. Nazi Germany started the war in Europe only after its economy had recovered. Not all the countries affected by the Great Depression were taken over by fascist regimes, nor did all such regimes start wars of aggression. In fact, no general relationship between economics and conflict is discernible for the century as a whole. Some wars came after periods of growth, others were the causes rather than the consequences of economic catastrophe, and some severe economic crises were not followed by wars.

In this excerpt, Ferguson argues that economic decline is neither a necessary nor sufficient cause of war: some wars followed economic declines but others did not, so no causal relationship can be demonstrated.

But what if it was true that every war was preceded by economic decline? Would that alone be enough to support a debaters’ claim that “economic decline causes war”? Not if economic decline is only necessary but not sufficient: while it may be true that wars will not occur in the absence of economic decline, it does not therefore follow that economic decline must necessarily cause war. Debaters often miss this crucial distinction: too often “if X, then Y is possible” becomes “if X, then Y for sure”.

In a 2009 article in Foreign Policy, Ferguson advanced a different argument about economic decline that provides another helpful example:

The Bush years have of course revealed the perils of drawing facile parallels between the challenges of the present day and the great catastrophes of the 20th century. Nevertheless, there is reason to fear that the biggest financial crisis since the Great Depression could have comparable consequences for the international system.

For more than a decade, I pondered the question of why the 20th century was characterized by so much brutal upheaval. I pored over primary and secondary literature. I wrote more than 800 pages on the subject. And ultimately I concluded, in The War of the World, that three factors made the location and timing of lethal organized violence more or less predictable in the last century. The first factor was ethnic disintegration: Violence was worst in areas of mounting ethnic tension. The second factor was economic volatility: The greater the magnitude of economic shocks, the more likely conflict was. And the third factor was empires in decline: When structures of imperial rule crumbled, battles for political power were most bloody.

…

Economic volatility, plus ethnic disintegration, plus an empire in decline: That combination is about the most lethal in geopolitics. We now have all three. The age of upheaval starts now.

Ferguson’s argument is that economic volatility, ethnic disintegration, and an empire in decline are all necessary conditions of global war. Individually, however, none of these are sufficient causes of global conflict. But because the world already includes both ethnic disintegration and an empire in decline, economic volatility is both a necessary and sufficient condition of war.

In this way, debaters can use their understanding of causality to further bolster the credibility of their internal link chains. Suppose that an affirmative team has argued that their plan is necessary to prevent further economic decline and that further economic decline would cause global war. In response, the negative argues that economic decline alone is not sufficient to cause war. Using Ferguson’s arguments as evidence, the affirmative can reply by contending that while it is true that economic decline alone is not a sufficient cause of war generally, it is a sufficient cause of war given the presence of the two other necessary conditions (ethnic disintegration and an empire in decline).

Mastery of the concept of causality as it is applied to the social sciences can be an extremely helpful skill in a debater’s intellectual arsenal. By understanding the difference between necessary and sufficient conditions, a smart debater can both reduce the risk of their opponents’ scenarios and bolster the credibility of their own impacts.