cases, marijuana. However, synesthetes report that common stimulants, like caffeine and cigarettes do not affect the strength of their synesthesia, nor does alcohol. A very different theoretical approach to synesthesia is that based on ideasthesia. According to this account, synesthesia is a phenomenon mediated by the extraction of the meaning of the inducing stimulus. Thus, synesthesia may be fundamentally a semantic phenomenon. Therefore, to understand neural mechanisms of synesthesia the mechanisms of semantics and the extraction of meaning need to be understood better. This is a non-trivial issue because it is not only a question of a location in the brain at which meaning is "processed" but pertains also to the question of understanding epitomized in e.g., the Chinese room problem. Thus, the question of the neural basis of synesthesia is deeply entrenched into the general mind–body problem and the problem of the explanatory gap. Genetics Due to the prevalence of synesthesia among the first-degree relatives of people affected, there may be a genetic basis, as indicated by the monozygotic twins studies showing an epigenetic component. Synesthesia might also be an oligogenic condition, with locus heterogeneity, multiple forms of inheritance, and continuous variation in gene expression. While the exact genetic loci for this trait haven't been identified, research indicates that the genetic constructs underlying synesthesia are most likely more complex than the simple X-linked mode of inheritance that early researchers believed it to be. Further, it remains uncertain as to whether synesthesia perseveres in the genetic pool because it provides a selective advantage, or because it has become a byproduct of some other useful selected trait. Women have a higher chance of developing synesthesia, as demonstrated in population studies conducted in the city of Cambridge, England where females were 6 times more likely to have it. As technological equipment continues to advance, the search for clearer answers regarding the genetics behind synesthesia will become more promising. Although often termed a "neurological condition," synesthesia is not listed in either the DSM-IV or the ICD since it usually does not interfere with normal daily functioning. Indeed, most synesthetes report that their experiences are neutral or even pleasant. Like perfect pitch, synesthesia is simply a difference in perceptual experience. The simplest approach is test-retest reliability over long periods of time, using stimuli of color names, color chips, or a computer-screen color picker providing 16.7 million choices. Synesthetes consistently score around 90% on the