If I told you the letter “G” is red, you might be confused. But if you had what’s called grapheme-color synesthesia (GC), there’s a good chance you’d argue I’m wrong. That’s because many people who experience this phenomenon think that “G” is green. Grapheme-color synesthesia is one of the more common types. So is chromesthesia, the association of colors with sounds, which famously allows people to experience music as color. There are also weirder and wilder forms: some people experience tastes associated with certain words, while certain synesthetes attribute oddness or evenness to things.

Although people have known about synesthesia for centuries, there’s still a lot we don’t understand about it. We know that GC synesthetes make effortless, automatic, and consistent connections between letters and colors, but researchers are still investigating what might influence both the condition, and the exact associations that people make. But some recent results suggest a potential contributor: fridge magnets.

A recent paper in PLOS ONE suggests that the letter-color pairings of many GC synesthetes might have been conditioned by objects in their environment as children, such as colorful alphabet fridge magnets. The researchers are clear that they’re not suggesting that synesthesia can be learned, or that colorful letter toys lead to synesthesia. Rather, their results simply show that the associations can be influenced by their environments. Knowing this can shed light on some of the mysteries about the condition.

The data came from the Synesthesia Battery, a site where people who suspect they have synesthesia can explore their condition in exchange for providing research data. The tests on the site verify that participants are synesthetic by checking whether their letter-color matches are consistent, and whether they take longer to process letters that aren’t in the “correct” color.

Using data from 6,588 people, the researchers worked out which colors were most commonly associated with which letters. They found that their results were consistent with previous experiments showing that English speakers often have possible spelling-based associations. For example, “G” is commonly green, while “Y” is commonly yellow—tendencies that can be found in the trends present in large collections of synesthetes.

These results were then compared to the colors of a specific fridge magnet set, a Fisher-Price set that was produced between 1971 and 1990. The pool of participants was narrowed down to those born after 1967. Out of the resulting 6,232 GC synesthetes, 388 (5.9 percent) had more than 10 letter-color associations matched to the magnet set. For those born after 1971, this proportion rose to 9.1 percent, and for those born between 1975 and 1980, it reached as high as 15 percent.

In comparison, the overall population were more likely to match the general associations found in the full sample of 6,588 people. Nearly half of all the GC synesthetes, with birthdates ranging from 1940 to 2000, had over seven matches to the more general pattern (such as the "G" is for green example above). “Whatever drives the [general] matching behavior in the data set has been continuously present, while the proportion of magnet synesthetes rises and falls with availability of the magnet set,” write the authors. When the “magnet” synesthetes had some associations that didn't echo the magnet set, they generally fell in line with the overall pattern.

What’s important to remember is that these are just the cases we can trace, the authors note. This was a particularly popular toy available during a set time period—for other synesthetes, there could be more obscure influences, like a particular alphabet poster in a kindergarten classroom. Each individual might also have associations influenced by a specific toys or other factors. And of course, there also are the cultural influences like the linguistic cues leading to a common association between “B” and “blue”.

What this tells us is that GC synesthesia can, at least sometimes, be shaped by environmental factors. This finding doesn’t contradict data showing that synesthesia is linked to genetic predisposition, the authors write, but it does tie in with other evidence suggesting that learning plays a role in certain kinds of synesthesia. That seems to be true for most synesthesia that involves some kind of culturally determined sequence, like days or the week or numbers; other kinds of synesthesia, like chromesthesia, may not operate in the same way.

One question raised by this finding is whether the designer of the magnet set was a GC synesthete, and whether this means it’s possible for synesthetic associations to be culturally transmitted. There’s also more work to be done establishing the exact link between synesthesia and associative learning, working out why some, but not all, associative learning is linked to synesthesia.

PLOS ONE, 2015. DOI: 10.1371/journal.pone.0118996 (About DOIs).