How killing begets more killing (of bugs)

"At first killing was obligatory; afterward we got used to it. We became naturally cruel. We no longer needed encouragement or fines to kill, or even orders or advice." Jean-Baptise Murangira, a killer from Rwanda.
There's plenty of anecdotal evidence that killing escalates - that the more a person kills, the easier they find it to kill again. Andy Martens and Spee Kosloff have investigated this idea experimentally using a bug killing paradigm. Undergrad participants took part in what they thought was a study looking at the ways that exterminators deal with bugs. In fact no bugs were hurt during this research and participants were debriefed "sensitively" at the end. Three participants dropped out after reading the consent form and a further three were omitted from analysis because they suspected they weren't really killing the bugs.

The fifty remaining participants (29 women) were each left in a room alone and given 12 seconds to feed as many of the 21 available woodlice as they wanted, one at a time, into an extermination machine - an adapted coffee grinder with a funnel for inserting the bugs. The woodlice had been warmed briefly with a hair dryer so that they were moving about, thereby ensuring that participants knew they were alive. After the first 12-second phase, the researcher returned to the room briefly to tell the participants that they had a further 12 seconds to repeat the exercise anew. The key test was whether the participants would kill more bugs on the second round.

Crucially, half of the participants had been told that the whole thing was merely a simulation - no bugs would die in either round because the funnel was blocked. These participants inserted the same number of bugs on the second round as they did on the first round (approximately 5.5 bugs on average). By contrast, the other participants who'd been told that the killing was real tended to insert more bugs into the exterminator during the second round (5.4 bugs on average vs. 4.6 on average during the first round).

Martens and Kosloff said this provides evidence that killing escalates. The fact that the escalation was only observed for the participants who thought they were killing shows that the effect wasn't merely due to practice. Earlier research similarly showed that participants instructed to kill more bugs in an initial phase (vs. those told to kill fewer) tended to kill more bugs voluntarily in a second phase - but only if they believed the killing was real.

Why should killing escalate in this way? Martens and Kosloff's theory is that it probably has to do with the justification of earlier actions and with desensitisation. Killing may particularly provoke these reactions in killers because unlike other unethical acts, it isn't possible to undo an earlier killing and amends can't be made to the killed. Other possible factors that lead to escalation could include increased arousal and feelings of power and excitement.

"The impetus of this work was not to develop an understanding of bug-killing per se, but to aid an understanding of situations in which people kill other people," the researchers said. "Perhaps [results like these] can inform the generation of questions necessary for developing a well-rounded understanding of real-world consequences of killing and violence."


Martens, A., and Kosloff, S. (2012). Evidence that Killing Escalates Within-Subjects in a Bug-Killing Paradigm. Aggressive Behavior DOI: 10.1002/ab.21412

Post written by Christian Jarrett for the BPS Research Digest.
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