Sex vs. Reproduction Reproduction is essential –Brain is genetically hardwired to motivate us to reproduce Reproduce via sex Sex is pleasurable – motivated by pleasure Pleasure is proximate cause of sexual behavior (not reproduction) –Sex and reproduction can be separated –Pleasure mechanisms similar to other motivators such as drugs & eating

Why Humans have Sex Meston & Buss, 2007 –444 people provided 237 “different” reasons –1549 people rated described whether each reason described a reason they had sex (5 point scale – none to all sexual experiences) –Four major categories Pleasure, Goals, Emotion, Insecurity Pleasure can be linked to all four

Gender Motivation Differences Preference differences –Women Wealth (or potential of wealth) Higher social status Older Ambitious Dependable and committed Good parenting skills

Gender Motivation Differences Preference differences –Men Young Physically attractive

Gender Motivation Differences Reproduction –Species are motivated to reproduce In humans, sex does not equate to reproduction Raising infants requires huge investment in time and energy Healthy children that live to adulthood –Evolutionary Psychology Explanations Women are more limited than men in number of offspring Women – more weight to male characteristics that associated with raising children –Wealth, status, commitment Men – more weight to characteristics associated with health –Attractiveness (symmetry) & youth

Gender Motivation Differences –Differences are relative Men also value commitment –Mating with attractive & young women does not mean that children will survive Women also value physical characteristics (attractiveness, size, & athletic ability) –Some recent evidence suggests men with these characteristics are more attractive when women are ovulating

Sex Hormones Androgens (testosterone) –Males levels are 8-10x higher –Generally increases size – muscles, bones, hearth, etc. –Essential for male embryonic development Estrogens –Female levels are 6-10x higher (vary considerably across the menstrual cycle – highest just before ovulation) –Leads to female sexual characteristics – breasts, hips Progestens –High levels after ovulation (implantation) and gestation, decreasing levels lead to menstruation

Sexual Orientation Heterosexuality, homosexuality, bisexuality –Homosexuality rates in humans probably between 3- 5% for males 1-2% for females Homosexuality occurs in many species –Homosexual behavior observed in over 1000 species –Other species show evidence of exclusive homosexuality & lifetime mating (e.g., penguins) –Prevalence across species suggests that homosexuality is associated with adaptive traits If simple genetic trait, it would disappear because homosexual animals less likely to reproduce

Sexual Orientation Known “causes” –No single cause Likely are many contributing factors –Different causes for different individuals Complex traits such as sexuality are due to interactions between genetics & environment –May require a combination of genetic & environmental factors to occu –Genetics Heritability estimates between 30-70% (comparing monozygotic & dizygotic twins) –Birth Order Males only – each older brother increases likelihood of being homosexual (by approximately 33%) –E.g., 1 st – 1.8%, 2 nd – 2.4%, 3 rd – 3.2% –Possible cause might be mother’s antibody reaction to male antigen that is important for sexually differentiating brain (e.g., H-Y antigen)

Sexual Motivation Theory & research in sexual motivation lags other biological motives –Research with humans more limited Accuracy of self reports, random assignment, etc. –Social interaction Much harder to study behaviors that require cooperation between two individuals –Important differences with many animals that can be easily studied Majority of animals have sex only when female is fertile –Relative scarcity of early theories to spur research E.g. homeostasis works well for hunger & thirst but not sex

Sexual Motivation Sexual motivation may be similar to other “biological” motives such as food, thirst, & drugs –Like food there is evidence that endogenous opiate (liking) & dopamine (wanting) systems Endorphins/enkephalins (natural opiates) released during sex E.g., Dopamine agonist lead to erection & DA antagonist leads to reduced sexual desire –Systems triggered by environmental stimuli Images, smells, thoughts, touch, etc. Activation of liking & wanting systems “Preparatory” physiological responses (changes in blood flow, heart rate, etc.)

Factors that increase Motivation Testosterone –Generally related to sexual desire in both men and women Men - greatest in early adulthood and gradually decreases with age (as does sex drive) Men – testosterone fluctuates with female cycle (greater at ovulation); due to unknown pheromones Women - fluctuates across menstrual cycle in women and women show greater sexual interest when high –Works more as an enabler than driver of sexual motivation Arousal –Arousal can heighten dominate reaction such as attraction to another (bridge study)

Functions of Sex Reproduction –Regular (weekly) sex increases likelihood of fertilization – higher estrogen levels and more regular menstrual cycles Create bond between two people –Pleasure of sex may help create strong & lasting connections that are necessary for successfully raising children –Classical conditioning Association with pleasure and stimuli (partner) –Animals show strong preferences for locations where they have had sex, smells associated with sex, etc. CC may explain other “non-reproductive” sexual interests (i.e., fetishes)

Functions of Sex Create bond between two people –Oxytocin NT in brain & blood hormone that is important in birth and lactation Released during orgasms May contribute to trust & bonding –Important for mate bonding in at least one species –Nasally administered oxytocin produced higher partner trust in investment game (only when they thought partner was human) –Ecstasy may indirectly stimulate oxytocin (ecstasy can produce feelings of love & connection)