ancient China where foot binding was practiced. In Jewish Rabbinic literature, the rabbis considered small feet to be the ideal type of feet for women. Hair Men have been found to prefer long-haired women. An evolutionary psychology explanation for this is that malnutrition and deficiencies in minerals and vitamins causes loss of hair or hair changes. Hair therefore indicates health and nutrition during the last 2–3 years. Lustrous hair is also often a cross-cultural preference. A component of the female beauty ideal in Persian literature is for women to have black hair, which was also preferred in Arabian society in the Middle Ages. In Middle English literature, curly hair is a necessary component of a beautiful woman. Fertility-driven attractiveness There are some subtle changes in women's perceived attractiveness across the menstrual cycle. During their most fertile phase, we can observe some changes in women's behavior and physiology. A study conducted by G. Miller (2007) examined the amount of tip earnings by lap dancers across the menstrual cycle. He found that dancers received nearly US$15 more when they were near ovulation than during the rest of the month. This suggests that women either are more attractive during ovulation phase, or they experience a significant change in their behavior. Some other studies have found that there are subtle differences in women's faces when in their fertile phase. Bobst and Lobmaier (2012) created 20 prototyped photographs, some of a female during ovulation and some during the luteal phase. Men were asked to choose the more attractive, the more caring and the more flirtatious faces. They found a significant preference for the follicular phase (ovulation). This suggests that subtle shape differences in faces occurring during the female's ovulation phase are sufficient to attract men more. This idea is supported by another study, where a similar experiment was done. Men and women had to judge photographs of women's faces taken during their fertile phase. They were all rated more attractive than during non-fertile phase. There are some subtle visible cues to ovulation in women's faces, and they are perceived as more attractive, leading to the idea that it could be an adaptive mechanism to raise a female's mate value at that specific time (when probability of conception is at its highest). Women's attractiveness, as perceived by men and women, slightly differs across her menstrual cycle, being at peak when she is in her ovulation phase.