Sexual selection

Sexual selection influences reproductive success. Intra-sexual selection refers to memers of the same sex competing with each other to access to members of the other sex. Those that succeed (victors) have the opportunity to pass on their genes, whilst the losers do not have this opportunity. Traits that lead to success in such competitions will be passed on to the next generation. Inter-sexual selection refers to preference for certain qualities in members of the opposite sex. Such preferences determine the area of competition amongst the opposite sex. Indicators of such traits would help in the selection of partners who would be able to protect and support offspring, as well as identifying traits that would be passed on to offspring and which would increase chances of survival. 

Buss (1989) surveyed over 10,000 people from 37 different cultures which were diverse in terms of ethnicity, religion and social class. Women desired mates who had good fiancial prospects, indicating ambition which aids resource acquisition. Men placed more importance on physical apperance, and younger partners, as indicators of fertility and younger women. Both sexes wanted intelligent, kind and dependenable partners, indicating abilility to protect, parenting skills and an interest in a long-term relationship.

Such explainations for mate selection and reproductive behaviour are deterministic, proposing the traits we look for are predetermined. This fails to accound for any choice in relation to traits desirable in a partner. Women who have their own finiancial resources might choose a partner that can provide emotional resources instead. Equally, some make the choice to avoid reproduction, which would not be adaptive and enble genes to be passed on. 

Short term preferences: Men has evolved a preference for casual sex, early on in a relationship. This would be an advantage as it would enable him to pass on more copies of his genes. It would not be advantageous for women to have several sexual partners as she is only able to produce one child in the same amount of time. Men have lower standards than women with regards to short-term mating preferneces , and then show a marked decrease in attraction following sex. This would have evolved to enable them to make a hasty departure and find their next sexual partner. 

Supporting this, Clark and Hatfield conducted an experiment in which male and female experimenters approached strangers on a college campus, saying 'Hi I've noticed you around campus and I find you every attractive.' They then asked them if they would go on a date, come back to their apartment or have sex with them. 50% of females agreed to go on a date, 5% agreed to go back to their apartment and 0% agreed to have sex with them. 50% of males agreed to go on a date, 69% agreed to go back to their apartment and 75% agreed to have sex. This supports the idea that men have evolved a prefernece for sexual variety and little time between partners to succeed in short-term mating. 

One issue related to short-term mate preference is a gender bias (alpha bias) suggesting only males desire a variety of sexual partners. Men could never have evolved a preference for a variety of sexual partners, without having willing females. Perhaps, women do also have such preferences as it may be adapative to use short-term mating as a way of leaving a poor-quality relationship, which may prevent reproductive success. 

Such studies pose ethical issues and may lack validity due to social desireability (expand)

Long-term mating preferences, both men and women have higher standards when it comes to selecting a long-term partner, as it they make a poor choice, they will waste valuable resources. Women are attracted to men who are able to invest resources in her and her childern, are able to physically protect her and her childern, show promise as a good parent and are sufficiently compatible, to minimise costs to her and her offspring. Men would be more likely to be attracted to women who show signs of fertility to ensure reproductive success. 

Miller et al (2007) calculated the tips earned by lap dancers at varying stages of their menstrual cycle. Those girls who were in the fertile oestrus phase of their cycle (most fertile) earned almost twice the value in tips, compared to girls who were not in oestrus. This supports the idea that men hae a preference for fertility. 

The idea of sexual selection influencing reproductive behaviours supports the nature side of the debate, suggesting such mechanisms are innate. This ignores the fact that partner selection may be learned and indeed, influenced by nurture. It may be the case that men who have ambition and financial resources may not satisfy emotional needs of a women. If such a relationship was not rewarding, women may seek different traits in a partner. 

The menstrual cycle can have an affect on both short-term and long-term mate preferences, suggesting the mate choice of females is variable. Penton-Voak et al (1999) found that women preferred a feminised version of a male face for a long-term relationship. However, preferences for more maculine faces were identified in relation to short-term partners, when there is high risk of conception during the menstrual cycle. This may be adaptive to secure long-term partners who will possess good parenting skills and short-term partners who will pass on characteristics such as an efficient immunes system (high level of testosterone suppress the immune system and therefore if such individuals are in good health, their immune system must be efficient).