Negotiating with terrorist kidnappers significantly increases the likelihood of further kidnappings, researchers at the University of Texas at Dallas found in a new study that will be published next month.

UTD professors Todd Sandler and Patrick Brandt looked at data on transnational kidnappings from 2001 to 2013 and found that successful negotiations meant a 66-87 percent increase in the likelihood of further kidnappings.

“We provide strong evidence that terrorist negotiation success results in more hostages being abducted because of terrorists’ anticipated future payoffs, consistent with our conceptual model’s predictions,” the authors noted.

The study also found that casualties suffered by terrorists did not deter future kidnapping attempts, a result the authors chalked up to the “religious fanatacism” of many terrorists.

“Given the religious fanaticism of many of today’s terrorist kidnappers, it is not surprising that terrorist casualties had no significant deterrent effect,” the authors noted.

The study, entitled “Why concessions should not be made to terrorist kidnappers,” will appear in the September 2016 edition of the European Journal of Political Economy.

The Obama administration is facing increased scrutiny after admitting that a $400 million cash payment to Iran was contingent upon the release of four American prisoners held hostage by the country. U.S. State Department officials said the payment was used as “leverage” in negotiations with the Iranians.

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