take three or four. Some mice are simply more sensitive to pain than others. Since the environmental conditions are apparently just the same, it is tempting to conclude that differences in this simple behavior reflect some difference in the mice’s genes. It’s an easy enough question to check: Since the experiments are performed on different genetic strains of mice, all you need to do is compare the results for the different strains to see if some genetic profiles corresponded with slower (or faster) tail-jerk reactions than others.

As it turns out, Jeffrey Mogil of McGill University in Montreal and collaborators at the University of Illinois had been dipping mouse tails in hot water for more than a decade and had accumulated plenty of data with which to perform such an analysis. And that analysis did confirm the relevance of genetic differences. Keep the environmental conditions constant (the water temperature should be precisely 49 degrees Celsius, for example) and some genetic strains, on average, do flip their tails out of the water faster than others.

Upon further review, though, it became clear that genes were not the only things that mattered, and a constant water temperature was not the only environmental factor to consider. After reviewing the scores of more than 8,000 irritated mice, Mogil’s team found that all sorts of things influence reaction speed. Are the mice kept in a crowded cage, or do they have room to roam? Was it the first mouse out of the cage, or the second? Is it morning, afternoon, or night? Did anybody remember to measure the humidity? And who was holding the mouse at the time? “A factor even more important than the mouse genotype was the experimenter performing the test,” Mogil and colleagues wrote in their paper.20 In other words, genes aren’t even as important as which researcher is handling the mouse.

In fact, a computerized cross-check of all the factors found that genetic differences accounted for only 27 percent of the variation in tail-test reaction speed. Environmental influences were responsible for 42 percent of the performance differences, with 19 percent attributed to interactions between environment and genes.

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