There's quite a lot of literature on the US and UK's no-concessions policies on kidnapping. Here's one example [1]. A few quotes:
> Despite the U.S. no-concessions policy, U.S. citizens continue to top the list of nationalities kidnapped by terrorists. This may be explained by the prominent role and perceived influence of the United States and the ubiquity of U.S. citizens around the world. Nationals of the United Kingdom, which also has a no-concessions policy, are second on the list.
> While a no-concessions policy may not deter kidnappings, it may affect the treatment of hostages in captivity and determine their ultimate fate. According to a 2015 study published by West Point, Americans held hostage by jihadist groups are nearly four times as likely to be murdered as other Western hostages (Loertscher and Milton, 2015). The no-concessions policy may be only part of the reason. Another factor would be the jihadists’ intense hostility toward the United States.
> While the U.S. no-concessions policy has not deterred
kidnappings, there is some evidence that political concessions and ransom payments appear to encourage further kidnappings and escalating demands.
> And although it did not produce any demonstrable decline
in kidnappings of U.S. citizens, a 2016 study published in the European Journal of Political Economy argues that, without the no-concessions policy, there would have been even more kidnappings of U.S. nationals (Brandt, George, and Sandler, 2016).
My take: Arguably, part of the reason the policy has not been successful in preventing kidnappings is that most of Europe does pay ransoms, and Europeans versus Americans are not always easily distinguishable. Even if the policy hasn't directly stopped kidnappings, it probably has stopped them indirectly, by avoiding funding kidnapping organizations. Europe has spent hundreds of millions of dollars in ransoms to terrorist organization, and Qatar allegedly paid close to a billion dollars in ransom. This has to fund further efforts.
Additional literature on the topic[0]. The finding is that any payment at all is sufficient for the operation to continue. This makes sense for ransomware was well since the marginal cost of hacking additional targets is effectively zero.
The major ransomware operations are targeted and the hackers do research the victims. They use spear phishing, so they need to know their victim. Unless the ban is universal and consistent so that hackers can modify their behaviour before they hack a target, there is no point in doing it. The US treasury announcement about not paying ransoms is just such a pointless terrible idea.
> Despite the U.S. no-concessions policy, U.S. citizens continue to top the list of nationalities kidnapped by terrorists. This may be explained by the prominent role and perceived influence of the United States and the ubiquity of U.S. citizens around the world. Nationals of the United Kingdom, which also has a no-concessions policy, are second on the list.
> While a no-concessions policy may not deter kidnappings, it may affect the treatment of hostages in captivity and determine their ultimate fate. According to a 2015 study published by West Point, Americans held hostage by jihadist groups are nearly four times as likely to be murdered as other Western hostages (Loertscher and Milton, 2015). The no-concessions policy may be only part of the reason. Another factor would be the jihadists’ intense hostility toward the United States.
> While the U.S. no-concessions policy has not deterred kidnappings, there is some evidence that political concessions and ransom payments appear to encourage further kidnappings and escalating demands.
> And although it did not produce any demonstrable decline in kidnappings of U.S. citizens, a 2016 study published in the European Journal of Political Economy argues that, without the no-concessions policy, there would have been even more kidnappings of U.S. nationals (Brandt, George, and Sandler, 2016).
[1] https://www.rand.org/content/dam/rand/pubs/perspectives/PE20...
My take: Arguably, part of the reason the policy has not been successful in preventing kidnappings is that most of Europe does pay ransoms, and Europeans versus Americans are not always easily distinguishable. Even if the policy hasn't directly stopped kidnappings, it probably has stopped them indirectly, by avoiding funding kidnapping organizations. Europe has spent hundreds of millions of dollars in ransoms to terrorist organization, and Qatar allegedly paid close to a billion dollars in ransom. This has to fund further efforts.