Do they? To answer the question, consider two recent summaries of the relevant literature. The first comes at the outset of "When Do Economic Sanctions Work? Asymmetric Perceptions, Issue Salience, and Outcomes," by Adrian U-Jin Ang and Dursun Peksen, found in Political Research Quarterly March 2007 edition.
[T]he conventional wisdom appears to be that sanctions are ineffective and failed policy instruments in the vast majority of cases (Galtung 1967; Wallensteen 1968; HSE; Pape 1997, 1998; Drury 1998; Elliott 1998). Others, however, have dissented from the conventional wisdom and have been critical of the assessment of sanctions being simply a dichotomous success-failure measure (Daoudi and Dajani 1983; Baldwin 1985; Baldwin and Pape 1998). They argue that compliance ought not to be the sole criterion for judging the success or failure of sanctions. In most of the cases, even though the total compliance of targets may not have been obtained, the sender may have managed to wring significant concessions from the target or succeeded in achieving less ambitious foreign policy goals such as symbolic gains.It's important to realize the multifarious purposes of sanctions before determining they're a wholesale failure.
Lindsay (1986) undertook an examination of nineteen cases of trade sanctions and identified five separate foreign policy goals of senders: compliance, subversion, deterrence, international symbolism, and domestic symbolism. His findings suggested that sanctions aimed at compliance, subversion, and deterrence fail generally and that states often resort to sanctions for symbolic purposes.Is that all that sanctions are good for? Not so fast:
[M]ore recent studies in the literature demonstrated that assessments of sanctions effectiveness have neglected the threat of sanctions, which has resulted in a selection bias (Smith 1996; Drezner 1999, 2003; Miers and Morgan 2002; Nooruddin 2002; Lacy and Niou 2004; Y. Li and Drury 2004; Drury and Li 2006). These studies argue that sanctions succeed more often than commonly suggested once the cases in which coercion is threatened but not imposed are also included in the analysis. The assumption is that if the targets expect that they will change their policies as a result of the imposition of sanctions, they may prefer to capitulate to the sender at the threat stage to avoid the economic cost of implemented sanctions. Due to the absence of comprehensive data on threatened but not imposed sanctions, these studies, however, have been limited mostly to game theoretic models and case studies that should be the subject of further empirical inquiry.Limitations and qualifications: the bane of empirical research everywhere.
We turn to another article to see if Ang and Peksen's analysis is representative: Jon Hovi, Robert Huseby, and Detlef Sprinz's "When Do (Imposed) Economic Sanctions Work?" found in the July 2005 edition of World Politics. Their overall assessment is similar:
The dominant view historically has been that sanctions do not work. From Galtung's analysis of the sanctions against Rhodesia to Doxey's broader set of case studies, negative assessments have been numerous. According to Baldwin, "[i]t would be difficult to find any proposition in the international relations literature more widely accepted than those belittling the utility of economic techniques of statecraft."What keeps sanctions, in general, from succeeding?
First, it is difficult to ensure that sanctions hurt where they are supposed to hurt. For example, when sanctions are imposed unilaterally, the target might reduce their impact by turning to alternative customers or suppliers and by using counterstrategies such as stockpiling, import substitution, rationing, and smuggling ("sanctions busting"). Moreover, the political elite in the target country might be able to pass on the costs of sanctions to other segments of the population. second, sanctions can be costly for the sender, too. In particular, when trade sanctions are being used, the target's neighbors often suffer significantly. Finally, while sanctions might cause protest against the political leadership in the target state, they might also conversely arouse defiance, patriotism, and popular support for the regime. In some cases the latter effects outweigh the former, with the result that resistance is reinforced rather than reduced.At this point, the pragmatic rationale for sanctions seems pretty weak. Why do nations continue to employ them, then?
One answer is that sanctions also have domestic and symbolic dimensions. For example, sanctions might be imposed or sustained primarily to satisfy a domestic interest group or simply to demonstrate that the government cares and "is doing something." Others have emphasized that one needs to distinguish between (1) cases where sanctions have actually been imposed and (2) cases where sanctions have merely been threatened. Sanctions are usually threatened before they are imposed, and they are imposed only if the target refuses to comply. But if a credible threat of sanctions fails, it is usually a sign that the target does not intend to comply even if sanctions are imposed. So it is a curious fact that when sanctions are imposed, there are often good reasons to expect them to fail.This leads the author to be critical of research focusing only on cases where sanctions were actually imposed. As they argue,
A satisfactory data set should also include cases where sanctions were threatened but were not imposed. This type of data set is used by Drezner to analyze cases in which the United States threatened sanctions to achieve reduced trade barriers, compliance with labor standards, or protection of the environment. He finds a considerably higher success rate in cases that ended at the threat stage than in cases where sanctions were actually imposed.For the reasons cited above, this result makes sense: the most effective deterrent is the one that never has to be used.
So, to sum up: the Affirmative is on fairly solid ground to argue that sanctions are ineffective. In fact, I bet some Affirmatives are going to try to lay an "effectiveness burden" on the Negative. The Neg has at least two possible responses: adopting the burden and using the logic and evidence of deterrence, or rejecting the burden and arguing that even if economic sanctions are often ineffective, they're defensible as a morally superior or less wasteful alternative to war.