Evidence of Sexual Selection for Evening Orientation in Human Males: A Cross Cultural Study in Italy and Sri Lanka

Previous research has established the existence of individual differences with regards to individuals’ optimum time of well-functioning; specifically in terms of being either morning or evening oriented. An association has also emerged between being more evening, as opposed to morning, oriented and having a greater number of sexual partners. The aim of the present study was to investigate whether "eveningness” in males is an evolved sexually dimorphic trait consistent across different cultures. A sample of 179 male Sri Lankan men residing in two different cultural and economic settings, Italy and Sri Lanka, were administered the Morningness-Eveningness Questionnaire (MEQ) followed by assessing their sexual behavior history. The results robustly portrayed a highly significant main effect of MEQ types highlighting the twofold sexual success enjoyed by the evening individuals in both regional locations. Morning oriented individuals, showed a stronger preferece for going out and partying than evening-types, suggesting that the higher mating success of evening types is not due to their different lifestyles allowing more opportunities to encounter females. However, evening types exhibited a preference for flirtatious behaviors in the later part of the day. Shoulder-to-hip and handgrip strength, as measures of testosterone levels, were not significantly associated with eveningness. The results are discussed in terms of sexual selection and its interplay with human cultural variation.