Just as we don't have a solid grasp of synesthesia's cause, we also don't have a solid grasp of its prevalence. According to Scientific American, it may affect anywhere from one in 20,000 people to one in 200. Synesthesia, to be clear, is different than hallucination. While hallucinations have no predictable pattern, synesthetic sensations happen the same way every time: A particular synesthete may see every number 2 as orange, or taste coffee every time they hear the note F. The experiences are unique to each synesthete. If two people see every number in its own color, one might see 5 as red while the other may see the same number as blue. Likewise, the sensations aren't linked to meaning: for the same person, the digit version of a number may take on a different color than the word spelled out.
Here's what we do know: it tends to run in families, and nearly six times as many women experience the condition as men. Researchers have found certain genetic markers associated with synesthesia. Interestingly, those markers are also strongly linked to autism—which could explain why so many people with autism also have synesthesia—as well as to epilepsy. Experts also think that we may all be born with synesthesia, but most of us lose those brain connections as we develop.