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The two pig populations monitored by the researchers, one each in Denmark and Sweden, showed that aggressiveness is affected by genetics to a similar extent as other traits currently included in selection programs. The number of skin lesions 24 hours after mixing showed similarly moderate levels of heritability in both Danish and Swedish pigs. Fighting behaviour itself was also moderately heritable, while bullying was only slightly less so. Being the target of bullying, on the other hand, was shown to be only mildly heritable.
http://www.welfarequality.net/everyone/42323/7/0/22
