Answer:
The causes of synesthesia remain unknown. Some scientists have suggested that everyone is born synesthetic but that the typical developmental trajectory results in these highly interconnected brain areas have become far more segregated. We do not know why synesthetes retain some of these anomalous connections. A biological determinant may be partially at work in certain cases of synesthesia, because the condition tends to run in families; moreover, nearly six times as many women as men report synesthesia. Whatever its causes or origins, synesthesia provides cognitive neuroscientists with a unique opportunity to learn more about how the brain creates our perceptual reality.
Synesthesia is a relatively rare condition, reported to occur in 1 in every 250 to 25,000 people, although these estimates are not based on good scientific evidence and many researchers believe it is more common than previously stated. It is probably best thought of as a family of related conditions. Different neurological mechanisms may be responsible for different types of synesthesia.