Transcript Document
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 35% 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., 1st – 1.8%, 2nd – 2.4%, 3rd – 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)