Another reason for homogamy is comfort level. Sexual preference creates a tendency towards assortative mating or homogamy. Heterophily also may be mentioned in areas such as homogamy, exogamy, and endogamy. Self-pollination is promoted by homogamy. Another reason people often chose homogamy is because of geographic proximity. The trends of socioeconomic homogamy are studied by the analysis of class, background and education. Homogamy and endogamy may be a result of cultural practices or personal preference. A small percentage of all marriages are inter-racial marriages (the opposite of homogamy). Marriage between partners of roughly similar age is known as "age homogamy". Therefore, same-sex relationships can fall under homogamy. Social homogamy refers to "passive, indirect effects on spousal similarity". This encourages homogamy and arranged marriages within the ethnic group.