George Soros built one of the great trading records in financial history, in large part due to his theory of reflexivity. But by his own account, some of his biggest portfolio pivots weren't triggered by a spreadsheet; they were triggered by backache.

It is a wonderful anecdote—Soros feeling a trade just as sailors feel a storm coming in their bones. But is it physiologically plausible? Fascinatingly, the evidence says yes. Here's why other investors should listen.

In Soros on Soros (1995), he noted, "I rely a great deal on animal instincts. When I was actively running the fund, I suffered from backache. I used the onset of acute pain as a signal that there was something wrong in my portfolio." His son Robert worded it in blunter terms: his father's elaborate market theories often arrived after the fact, rationalising a shift that had really begun with a spasm in his back.

When Soros experiences what he refers to as animal instincts (acute pain onset as a signal to act) he may actually be describing his experience of a classic physiological survival response—specifically, the reaction of the psoas muscle. Often nicknamed the fight-or-flight muscle, the psoas contracts when we feel under threat in some way, readying us to brace, fight, or........

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