As important as "organismic" factors (e.g., genetics, sex) are to pain-related variability, the lion's share of observed variability is actually produced by environmental factors. The most obvious of such factors is the animal's prior experience with pain, but even in pain-naïve animals, there is much non-genetic variability awaiting an explanation. A major new thrust in the laboratory is the identification of such factors in the laboratory environment. We have found that the largest factor is actually the…EXPERIMENTER him or herself, sort of a Heisenberg Uncertainty Principal of Pain Testing. Other factors that matter include: season, housing density, time of day, humidity, and within-cage order of testing.
A major finding in this domain from our laboratory is that pain sensitivity of mice is actually affected by the pain experience of their cagemates, suggestive of social communication-based pain modulation. We believe these data are best explained as "emotional contagion," which is a form of empathy. Based on this and follow-up findings from our laboratory, Ben-Ami Bartal and colleagues recently demonstrated prosocial or helping behaviour in rats, a yet higher form of empathy.