In 1892 Mary Whiton Calkins, a professor at Wellesley College, along with her experimental psychology class, canvassed the campus for a population that still intrigues us today: synesthetes. Synesthetes experience vivid impressions in one domain in response to stimuli in another, automatically perceiving the color yellow, for instance, upon seeing the letter y, attributing strong personalities to the names of weekdays, or imagining a distinct squiggle each time middle C is struck on the piano. Of the 525 people Calkins and her students queried, about 15 percent experienced some synesthesia.

More recent, stringent estimates put the proportion of synesthetes—whose associations are distinct, stable, automatic, and variable from one individual to the next—at around one in 20. (Even this proportion surprised me: apparently America may well have more synesthetes than it has farmers.)

Language researcher Karen Chenausky has described her experience growing up with synesthesia as “kind of like figuring out that you have a belly-button”: one first notices the synesthetic perceptions, then becomes obsessed with them, and finally moves on with one’s life. But my favorite descriptions of the phenomena come from Calkins’ early report. “T’s are generally crabbed, ungenerous creatures,” explains one of her participants. “Colors do not look right,” writes another, “unless a word is spelled right. For instance, I spelled permanent, the other day, with two a’s, and it did not look pale enough.” And from another participant: “For numbers, I entertain either a like or a dislike; for instance, 11, 13 and 17 are especially disliked, I suppose because they are prime. My feeling for 11 is almost one of pity.”

Why some people experience synesthesia while others do not remains something of a mystery, though genetics appear to play a role. As for understanding how individual synesthetes arrive at the pairings they do—why an a is perceived as red, for instance, and not blue—a recent study suggests a humdrum possibility.

A few years ago, Stanford researchers Nathan Witthoft and Jonathan Winawer published data demonstrating that one synesthete’s specific “grapheme-color” pairings (where graphemes refer to letters and digits) could be traced back to a set of colored Fisher Price refrigerator magnets from the participant’s childhood. Their data suggested that, at least sometimes, synesthetic pairs are learned not from intangible properties of graphemes, words, colors, or the mind, but from commonplace objects in the environment. The data tantalized, but didn’t convince: as I’ve written about in the past, psychologists tend to be a stodgy bunch when it comes to case studies.

But then other synesthetes, recognizing themselves in the study, contacted the researchers, and a second, larger study—published earlier this year—was born. In total, the researchers were able to round up 10 additional participants who seemed likely to have learned their grapheme-color associations from the Fisher Price set. (Indeed, all but one could recall owning the set as a child.)

Witthoft and Winawer first verified that the additional participants in fact met their criteria for experiencing synesthesia—crucially, they had to demonstrate pairings that were reliably more stable over multiple tests than non-synesthetics merely pretending to have synesthesia would be capable of. When researchers compared these participants’ pairings to the set of magnets, the match—particularly for the letters—was obvious: even the participant whose pairings deviated most from the magnets achieved a degree of similarity that would occur by chance less than one time in a billion. (Calkins—who favored a theory of “ordinary associations, probably of childhood” back in 1893—would have been quite pleased.)

Note that researchers are not arguing that the participants’ synesthesia was caused by the Fisher Price magnets, or that all synesthetic associations are a product of childhood environments. The synesthesia literature still abounds with patterns that nobody can really explain. “Depending on the survey, 33% to 40% of synesthetes have reported that the letter A is red, and 40% to 50% have said that Y is yellow,” Witthoft and Winawer write of earlier research. And indeed, when participants in the current study did deviate from the set of magnets, their deviations tended to match these often-reported patterns, suggesting multiple influences at work.

Odder still, the researchers note that these same patterns are actually “alignable with choices made by nonsynesthetes”—that is, we all to some extent intuit some of these relationships between colors and letters or words and shapes, just not to the same degree that synesthetes do.