While cross-sensory metaphors are sometimes described as «synesthetic», true neurological synesthesia is involuntary and occurs in slightly more than four percent of the population (1 in 23 persons) across its range of variants (Simner et al. in press). It runs strongly in families, possibly inherited as an X-linked dominant trait. Synesthesia is also sometimes reported by individuals under the influence of psychedelic drugs, after a stroke, or as a consequence of blindness or deafness. Synesthesia that arises from such non-genetic events is referred to as adventitious synesthesia to distinguish it from the more common congenital forms of synesthesia. Adventitious synesthesia involving drugs or stroke (but not blindness or deafness) apparently only involves sensory linkings such as sound ? vision or touch ? hearing; there are few if any reported cases involving culture-based, learned sets such as graphemes, lexemes, days of the week, or months of the year.

Although synesthesia was the topic of intensive scientific investigation in the late 1800s and early 1900s, it was largely abandoned in the mid-20th century, and has only recently been rediscovered by modern researchers. Psychological research has demonstrated that synesthetic experiences can have measurable behavioral consequences, while functional neuroimaging studies have identified differences in patterns of brain activation (for a review see Hubbard & Ramachandran 2005).