It is often mentioned that in BDSM, roles are not fixed to gender, but personal preferences. The dominant partner in a heterosexual relationship may be the woman rather than the man, or BDSM may be part of male/male or female/female sexual relationships. Finally, some people switch, taking either a dominant or submissive role on different occasions. Several studies investigating the possibility of a correlation between BDSM pornography and the violence against women also indicate a lack of correlation. As an example, Japan is listed as the country with the lowest sexual crime rate out of all the industrialized nations, despite being known for its distinct BDSM and bondage pornography (see Pornography in Japan). In 1991, a lateral survey came to the conclusion that between 1964 and 1984, despite the increase in amount and availability of sadomasochistic pornography in the U.S., Germany, Denmark and Sweden, there is no correlation with the national number of rapes to be found.