Birth order

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  • Both poppsychology and coursework have mentioned the birth order problem a lot, with no definitive answer. birth order has been popping up as an explanation for psychological variables for decades. It gains and then loses popularity as a causal variable for both intelligence and personality. It’s definitely an interesting and sort of “all encompassing” topic in that it relates to so many areas of psychology, including personality theory, intelligence theories, nature vs. nurture, and certainly neuropsychology. The whole debate can get messy, but also brings up the importance of empirical research and possible confounds. All of the researchers, primarily Rogers, Zajonc, are very passionate and adamant about their positions. -overview of the studies throughout history, from the beginning when it really focused on personality structure up to more current research which tends to focus on more of the biological or genetic aspects of birth order.
  • the individual who first drew popular attention to the birth order was Francis Galton, way back in 1874. Galton traced the lives of 180 “eminent men” and found that 48% were only or firstborn sons, 48% of course was highly significant. He concluded therefore that Firstborn children are overrepresented eminent men. However, conclusions may have been based on faulty reasoning, as those sampled were mainly in the sciences. More importantly, he didn’t count women at all. Therefore, an “ oldest” in his study may have had 9 older sisters and 1 younger brother.
  • The topic of birth order continued to be popular into this century and in the 60’s another surge of research began. These are some of the more well-known studies that tripped off the whole debate Altus, (1962) Firstborn male college students scored higher on verbal abilities than later born. Altus, et al., (1965) Birth order of students entering college at UC, Santa Barbara between 1960 and 1936 – over 60% were firstborn.
  • other well-known studies were conducted in the 70’s Belmont & Marolla, (1973) 386,114 Dutch men Data from Raven Progressive Matrices test Chilren from large families had poorer performance Firstborns scored better and gradient of declining scores with birth order Goertzel, Goertzel, & Goertzel, (1978) 314 eminent 20 th century individuals 46% were firstborn (social class controlled on Dutch study)
  • The research slowed during the 80’s, but some researchers were still looking at new data. Clark & Rice, (1982): Firstborns are overrepresented among Nobel Laureates. Terry, (1989): Firstborns are overrepresented among prominent psychologists. Armor, (2001): Examined data from over 2,000 families - 3 point difference between first and second born, 2 point difference between second and third born on scores of Peabody Picture Vocabulary Test.
  • So, if indeed there is a birth order effect, what is causing this all? one of the 3 mains theories of explaining the birth order effects is the admixture hypothesis. The Admixture Hypothesis Page & Grandon (1979): Parental IQ, SES, etc. may be responsible for both large families and low IQ, causing cross-sectional data to appear to support birth order effects. basically argues that birth order differences are simply artifacts of environmental variables and in fact birth order itself is not a causal variable. However, if this were true then the mean IQ score for any given population would gradually decline. That average IQ scores continue to increase goes against this theory.
  • Zajonc & Markus (1975): Firstborns are exposed to more adult language and attention in general. Laterborns are exposed to less mature speech and behavior, leading to less maturity with each additional sibling. Firstborns have the experience of tutoring, often being called upon to be “surrogate parents” therefore enhancing intellectual ability. Therefore, the confluence model predicts a negative influence of birth order for children less than 11 and positive for those over 11 years. This would explain the common only children do not have higher IQs than firstborns.
  • Another variable that is addressed by birth order studies is Family size. Large family size being related to lower IQ is consistently found across both cross sectional as well as longitudinal research. However, the causal of the relationship is arguable. Rogers, who is the major proponent that birth order does not have an effect, performed a study in 2000 of data from over 3,000 families. In the first paragraph his conclusions he boldly states: This study did cause quite a stir If you remember in 2000 on the cover of US NEWs there was a headline that low IQ mothers make low IQ children. Zajonc, again inventor of the confluence theory disputed the research methods that Rodgers used to find these results. The whole argument got very messy, with a lot of articles published in which they responded to one another and argued back and forth. - basically what researchers do agree on is that larger families show low IQ, but again the direction of causation is debatable.
  • Anther of the major arguments between Rodgers and proponents of birth order effects is whether you can study birth order cross-sectionally or if it has to be studied within families. Cross sectional studies tend to find significant birth order effects, particularly with IQ (Belmont & Marolla, 1973). Within family studies usually demonstrate no relationship between birth order and IQ (Berbaum & Moreland, 1980; Rodgers, et al., 2000).
  • Another direction researchers have gone in more recent studies is looking at personality traits and variables rather than intellegence among siblings. Personality variables, such as confidence or assertiveness, may account for say the success of 1stborns rather than the fact that they are smarter. Typically in this sort of research, you’ll find the opposite effect that you would with intelligence studies – in within family studies there are significant effects, but not in between-family studies. Incidentally, Sulloway published a book in 1996 called “born to rebel” claiming that in a nutshell firstborns are conformists and laterborns are rebels. The book was extremely popular and Sulloway claimed that the data collection and methods were very rigorous and scientific, based on data from thousands of individuals. However,others attempted to duplicate his results, they repeatedly could not come up with the same results, which casts doubt on the validity of Sulloway’s work. What you’ll typically find in pop psychology is that first borns are the perfectionists or people pleasers, middle borns are the social ones as they’re used to compromising and not being in the spotlight. The babies are the party animal, the entertainer, the thrill seeker. As I’ve shown though, there isn’t a lot of evidence for any of this in the literature. Not that its untrue, and just hasn’t been looked at as much as intelligence.
  • another variable that’s been studied is perfectionism – this is an important variable because it could explain the relative “eminence” or success of firstborns rather than intelligence Adler’s patterns: Firstborn: striving for perfection and need to please Middle: surrounded by competitors – peacemakers/arbitrators Youngest: inferiority – need to overcome everyone Only: center of attention/ atmosphere of parental anxiety
  • Also looking at birth order was Ashby, in 2003. They looked at perfection in a sample of undergraduates. Instead of using actual birth order, they used the psychological birth order Inventory o9r PBOI, developed in 91. Meaures a person’s perceived birth order position, which may or may not be actual order. Found in 1995 that there was a stronger relationship between psychological birth order and lifestyle than actual birth order and lifestyle. - Categories of adaptive perfectionists, maladaptive perfectionists, and non-perfectionists differ according to birth order. -Adaptive perfectionists were less likely to be middle children. -Adaptive and maladaptive perfectionists had fewer characteristics of younger children than non-perfectionists. -non-significant findings for oldest and only children
  • Cross-culturally, birth order research can look entirely different than that done in the US. Educated older siblings may provide more intellectual stimulation for youngers than would uneducated parents in Asian and African immigrant populations (Davis, Cahan, and Bashi). In a study of Kenyan adolescents, intelligence generally declined as birth order increased. However, 2 nd borns had higher IQ than first, attributed to demands facing firstborns (Munroe & Munroe, 1983). For this they used both school grades in English, math, and science, as well as visual and verbal tests, including block construction. Therefore, the dilution and confluence models may not work out cross-culturally
  • Columbian study of over 36,000 college applicants did NOT support dilution or confluence models. In fact, ranking in test scores by birth order was 3,4,5,2,6,1. Only 1% of variability was explained by birth order and family size. Socioeconomic status was a much more powerful variable. (used multiple regression)
  • This study, done in 2001, looked at Culture and Status Attainment 180 Anglo-Australian, 88 Greek, & 72 Italian young adults Longitudinal: data collected at age 11 (environmental influences on the family) Follow-up collection 5 years later on aspirations Data collection at 25 For Italians, only family size was negatively correlated with status. Birth order negatively correlated with status attainment for Anglo-Australians and Greeks (older had higher attainment).
  • Among a sample of Zimbabwean children, (511 boys and 632 girls), there was a general downward trend in IQ measures as birth order increased (Wilson, et al., 1990). A reason birth order research, if contributing little to psychology, may be beneficial in that it encourages families and governments to promote birth control and smaller family size, as the majority of studies do find this result.
  • Specific diseases such as epilepsy have also been looked at cross culturally. This study was done in Saudi Arabia, looking at epilepsy because there has been some thought that birth order might correlate with epilepsy. In the US, this study may inflate significant results because large families and later birth order are less common. In Saudi Arabia however, large families are very common and so there is a large population of later borns. Obeid, et al., 2002: 336 epileptics in Saudi Arabia Large family size is very common Mean family size is 6.8 Low birth order (older siblings) significantly more likely to have seizure disorder. Possibly due to prolonged labour in firstborns.
  • More than 1 child with autism Spiker, et al., 2001 144 Autism multiplex families Used Autism Diagnostic Interview – Revised (ADI-R) Lower nonverbal IQ for second born autistics / less than 50% of firstborns had nonverbal in ranges of retardation, but 70% of secondborns did. No relation to actual/normal’s birth order, but only autistic birth order One of the hypothesized reasons is that maternal antibodies are sensitized to fetal brain tissue and neurons Another thought is simply that there is decreased parental attention
  • another trend in research in more recent decades has been to look at more biological or physical, rather than factors that could be influenced by environment, such as personality. One such factor is lateral preference – whether an individual uses the right or left or is more ambidextrious. A school of thought in the lateral preference research is that there are some “natural” left-handers, but that some have suffered some sort of pathology. There was some thought that later order birth positions were more likely be left-handed – The Left Hemispherehas a greater need for Oxygen than the right and therefore is more susceptible to oxygen deficiency (Liederman, 1983). Therefore, children in either first born or later born positions are under more risk of undergoing trauma (Ernst & Angst, 1983). Some of the research did find significant results for birth order and lateral preference, however…this meta analysis did not find any relationship. Meta-analysis of studies on birth order, birth stress, and lateral preferences did not support relationship (Searleman, et al., 1989).
  • This is important research because it doesn’t include confounding variables, such as SES and is solely looking at biological factors. -this study was conducted in 2000, on 154 chimps -the “tube” procedure they spread peanut butter on the inside of PVC tupe that is long and narrow so the chimp has to reach inside to get at it. They look at which hand the chimp uses to do this Results showed that birth order has significant effect on lateral preference in captive chimps. First and later borns were significantly more likely to be left-handed -theorists came up with 2 theories as to why: number one is that first and laterborns are more likely to undergo birth trauma. A second explanation is that estrogen and testosterone can impact the developing cerebral hemisphere of the embryo and these hormones fluctuate systematically across birth order -another important part of the study is that they were able to control for maternal age, something not always done in birth order research, but it is extremely important as increased age of the mother correlates can correlated with a number of birth defects.
  • another direction the research has gone in recent years is looking at other variables such as sexuality. . Indeed the 30% was completely mistaken, but there was an actual empirical study on this when I looked further.
  • First major study was done by Blanchard in 1997
  • Reasons are somewhat similar to those hypothesized for chimps lateral handedness _one theory is that males provoke an immune response in the mother that is strengthened with each successive male birth – proposed antigen is supposed to be testosterone – -the mother’s antibodies are thought to pass to the fetus and affect the developing brain and affect sexual differentiation.
  • In 2004 Blanchard went on to do a “ Meta-analysis of aggregate data from 14 samples of 10,143 male subjects shows that homosexuality in human males is predicted by higher number of older brothers.” So this is pretty hard to argue with – the first thing I thought was “but how is this biological” -some support for it’s being a biological phenomenon is that heterosexual males with older borthers weighted less than those with older sisters -homosexual males with older brothers weighed less than those without older brothers -Also there has been evidence in other research that maternal antibodies may play a role in some neurodevelopmental disorders that happen more frequently in males, such as MR and learning disabilities -The competing theory is environmental – it proposes that sexual interaction with older males increases a males probability of developing homosexuality and a boy’s changes of this type of interaction increase with more older brothers. -Also an important point to remember is that this effect is not assumed to be the only factor playing a role in homosexuality. Using regression analysis to find the strength of the relationship between older brothers and homosexuality shows that the effect is definitely not the only factor responsible for homosexuality. It is estimated to account of aprox 30% for those males that are homosexual. In other words, only about a third may attribute homosexuality to the fraternal effect. Incidentally, prevalence of homosexuality among men is only at about 3%.
  • Birth order

    1. 1. Birth Order: History, Debate, and Current Research Brooke Schauder, PhD Erie Psychology Consortium Pacific Graduate School of Psychology
    2. 2. How it all began? <ul><li>Francis Galton “English Men of Science: Their Nature and Nurture” </li></ul><ul><li>Traced lives of 180 “eminent” men </li></ul><ul><li>Revealed that 48% were firstborn or only sons. </li></ul>
    3. 3. Classic Studies <ul><li>Altus, (1962) </li></ul><ul><ul><li>Firstborn male college students scored higher on verbal abilities than later born. </li></ul></ul><ul><li>Altus, et al., (1965) </li></ul><ul><ul><li>Birth order of students entering college at UC, Santa Barbara between 1960 and 1936 – over 60% were firstborn. </li></ul></ul>
    4. 4. Classics <ul><li>Belmont & Marolla, (1973) </li></ul><ul><ul><li>386,114 Dutch men </li></ul></ul><ul><ul><li>Data from Raven Progressive Matrices test </li></ul></ul><ul><ul><li>Chilren from large families had poorer performance </li></ul></ul><ul><ul><li>Firstborns scored better and gradient of declining scores with birth order </li></ul></ul><ul><li>Goertzel, Goertzel, & Goertzel, (1978) </li></ul><ul><ul><li>314 eminent 20 th century individuals </li></ul></ul><ul><ul><li>46% were firstborn </li></ul></ul>
    5. 5. Past to Present <ul><li>Clark & Rice, (1982): Firstborns are overrepresented among Nobel Laureates. </li></ul><ul><li>Terry, (1989): Firstborns are overrepresented among prominent psychologists. </li></ul><ul><li>Armor, (2001): Examined data from over 2,000 families - 3 point difference between first and second born, 2 point difference between second and third born on scores of Peabody Picture Vocabulary Test. </li></ul>
    6. 6. The Hypotheses <ul><li>The Admixture Hypothesis </li></ul><ul><li>Page & Grandon (1979): Parental IQ, SES, etc. may be responsible for both large families and low IQ, causing cross-sectional data to appear to support birth order effects. </li></ul>
    7. 7. Resource Dilution Model <ul><li>Blake, 1981: Parental Resources are Limited. With each sibling, attention, money, etc., must be further divided – diluting or decreasing the amount each sibling has. </li></ul>
    8. 8. Confluence Model <ul><li>Zajonc & Markus (1975): </li></ul><ul><ul><li>Firstborns are exposed to more adult language and attention in general. Laterborns are exposed to less mature speech and behavior, leading to less maturity with each additional sibling. </li></ul></ul><ul><ul><li>Firstborns have the experience of tutoring, often being called upon to be “surrogate parents” therefore enhancing intellectual ability. </li></ul></ul><ul><ul><li>Therefore, the confluence model predicts a negative influence of birth order for children less than 11 and positive for those over 11 years. </li></ul></ul>
    9. 9. Family Size <ul><li>Large family size correlates with lower IQ (Rodgers, et al., 2000). </li></ul><ul><li>“ Results of the analysis of the NLSY data support the belief the low-IQ parents make large families and are inconsistent with the belief that large families make low-IQ children.” </li></ul>
    10. 10. Cross Sectional vs. Longitudinal <ul><li>Cross sectional studies tend to find significant birth order effects, particularly with IQ (Belmont & Marolla, 1973). </li></ul><ul><li>Within family studies usually demonstrate no relationship between birth order and IQ (Berbaum & Moreland, 1980; Rodgers, et al., 2000). </li></ul>
    11. 11. Traits Vs. Intelligence <ul><li>Personality, in addition to or rather than intelligence, may vary by birth order (Simonton, 1999). </li></ul><ul><li>Conscientiousness and openness to experience may be much more important than IQ in terms of “eminence” (Sulloway, 1996). </li></ul>
    12. 12. Perfectionism <ul><li>Adler’s patterns: </li></ul><ul><li>Firstborn : striving for perfection and need to please </li></ul><ul><li>Middle : surrounded by competitors – peacemakers/arbitrators </li></ul><ul><li>Youngest : inferiority – need to overcome everyone </li></ul><ul><li>Only : center of attention/ atmosphere of parental anxiety </li></ul>
    13. 13. Ashby, et al., 2003 <ul><li>Study of 136 undergraduates completed Almost Perfect Scale – Revised (APSR) and PBOI. </li></ul><ul><li>- Categories of adaptive perfectionists, maladaptive perfectionists, and non-perfectionists differ according to birth order. </li></ul><ul><li>-Adaptive perfectionists were less likely to be middle children. </li></ul><ul><li>-Adaptive and maladaptive perfectionists had fewer characteristics of younger children than non-perfectionists. </li></ul><ul><li>-non-significant findings for oldest and only children </li></ul>
    14. 14. Cross-Cultural Research <ul><li>Educated older siblings may provide more intellectual stimulation for youngers than would uneducated parents in Asian and African immigrant populations (Davis, Cahan, and Bashi). </li></ul><ul><li>In a study of Kenyan adolescents, intelligence generally declined as birth order increased. However, 2 nd borns had higher IQ than first, attributed to demands facing firstborns (Munroe & Munroe, 1983). </li></ul>
    15. 15. (culture continued) <ul><li>Columbian study of over 36,000 college applicants did NOT support dilution or confluence models. In fact, ranking in test scores by birth order was 3,4,5,2,6,1. Only 1% of variability was explained by birth order and family size. Socioeconomic status was a much more powerful variable. </li></ul>
    16. 16. Culture and Status Attainment <ul><li>Marjoribanks, 2001 </li></ul><ul><li>180 Anglo-Australian, 88 Greek, & 72 Italian young adults </li></ul><ul><li>Longitudinal: data collected at age 11 (environmental influences on the family) </li></ul><ul><li>Follow-up collection 5 years later on aspirations </li></ul><ul><li>Data collection at 25 </li></ul><ul><li>For Italians, only family size was negatively correlated with status. </li></ul><ul><li>Birth order negatively correlated with status attainment for Anglo-Australians and Greeks (older had higher attainment). </li></ul>
    17. 17. Wisconsin Card Sort <ul><li>115 males and 104 females in grades 1-6 in Taiwan </li></ul><ul><li>Computerized WCST </li></ul><ul><li>No birth order effects found </li></ul>
    18. 18. (Culture Continued) <ul><li>Among a sample of Zimbabwean children, (511 boys and 632 girls), there was a general downward trend in IQ measures as birth order increased (Wilson, et al., 1990). </li></ul>
    19. 19. Epilepsy & Birth Order <ul><li>Obeid, et al., 2002: </li></ul><ul><ul><li>336 epileptics in Saudi Arabia </li></ul></ul><ul><ul><li>Large family size is very common </li></ul></ul><ul><ul><li>Mean family size is 6.8 </li></ul></ul><ul><ul><li>Low birth order (older siblings) significantly more likely to have seizure disorder. </li></ul></ul><ul><ul><li>Possibly due to prolonged labour in firstborns. </li></ul></ul>
    20. 20. Autism and Birth Order <ul><li>Spiker, et al., 2001 </li></ul><ul><li>144 Autism multiplex families </li></ul><ul><li>Used Autism Diagnostic Interview – Revised (ADI-R) </li></ul><ul><li>Lower nonverbal IQ for second born autistics / less than 50% of firstborns had nonverbal in ranges of retardation, but 70% of secondborns did. </li></ul><ul><li>Maternal antibodies and/or decreased parental attention </li></ul>
    21. 21. Lateral Preference and Birth Order <ul><li>The Left Hemispherehas a greater need for Oxygen than the right and therefore is more susceptible to oxygen deficiency (Liederman, 1983). </li></ul><ul><li>Therefore, children in either first born or later born positions are under more risk of undergoing trauma (Ernst & Angst, 1983). </li></ul><ul><li>Meta-analysis of studies on birth order, birth stress, and lateral preferences did not support relationship (Searleman, et al., 1989). </li></ul>
    22. 22. Lateral Preference & Birth order in Chimpanzees <ul><li>Hopkins & Dahl, 2000 </li></ul><ul><li>154 Chimpanzees, ranging from 3-57 years </li></ul><ul><li>Tube procedure </li></ul><ul><li>Results showed that birth order has significant effect on lateral preference in captive chimps. </li></ul><ul><li>Hormones’ effects on brain and/or trauma in first and laterborns </li></ul>
    23. 23. Sexual Preference and Birth Order <ul><li>“ With each older brother a male’s chances of being a homosexual increases by about 30% relative to a first born male.” </li></ul><ul><li>-Wickepedia </li></ul>
    24. 24. Blanchard, 1997 <ul><li>Homosexual men have a higher mean birth order than do comparable heterosexuals. </li></ul><ul><li>These results have been collected cross-culturally, in England, The Netherlands, Canada, and the US. </li></ul><ul><li>Results typically show that more older brothers, rather than sisters, is significant. </li></ul>
    25. 25. Fraternal Birth Order Effect <ul><li>Hypothesized mechanisms: “Maternal Immune response.” Supported in animal studies that maternal immune system may not recognize male hormones (Klassen, 1997). </li></ul>
    26. 26. Blanchard, 2004 <ul><li>“ Meta-analysis of aggregate data from 14 samples of 10,143 male subjects shows that homosexuality in human males is predicted by higher number of older brothers.” </li></ul><ul><li>Birth weight supports biological mechanisms. </li></ul><ul><li>Environmental theory that more older brothers increases odds of homosexual behavior. </li></ul>
    27. 27. Female Sexuality and Birth Order <ul><li>Bogaert, 1997 </li></ul><ul><li>Data on over 5,000 women was investigated. </li></ul><ul><li>No relationship between birth order and female homosexuality was found. </li></ul>
    28. 28. Bibliography <ul><li>Rodgers, Cleveland, Oord, & Rowe (2000). Resolving the debate over birth order, family size, and intelligence. American Psychologist, 55(6). </li></ul><ul><li>“ Birth Order” http://en. Wikipedia.org/wiki/Birth.order </li></ul><ul><li>Wilson, Mundy-Castle, & Panditji (1990). Birth order and intellectual development among zimbabwean children. Journal of Social Psychology, 130(3), 409-411. </li></ul><ul><li>Adams & Phillips (1972). Motivational and achievement differences among children of various ordinal birth positions. Child Development, 43, 155-164. </li></ul><ul><li>Velandia, Grandon, & Page (1978). Family size, birth order, and intelligence in a large South American sample. American Educational Research Journal, 15(3), 399-416. </li></ul>
    29. 29. (bibliography continued) <ul><li>Belmont & Marolla (1973). Birth order, family size, and intelligence. Science, 182, 1096-1101. </li></ul><ul><li>Galton, F (1874, 1895). English men of science: Their nature and nuture. New York: D. Appleton and Company. </li></ul><ul><li>Munroe & Munroe (1967). Birth order and intellectual performance in three East African Societies. Journal of Social Psychology, 123. </li></ul><ul><li>Altus, (1965). Birth order and academic primogeniture. Journal of Personality and Social Psychology, 2(6), 872-876. </li></ul><ul><li>Searleman, Porac, & Coren (1989). Relationship between birth order, birth stress, and lateral preferences: a critical review. Psychological Bulletin, 105(3), 397-408. </li></ul><ul><li>Ashby, LoCicero, & Kenny (2003). The relationship of multidimensional perfectionism to psychological birth order. Journal of Individual Psychology, 59(1). </li></ul>
    30. 30. (bibliography continued) <ul><li>Obeid, Awada, Amene, & Oni (2002). The controversy of birth order as a risk factor for epilepsy: a study from Saudi Arabia. Acta Neurol Scand, 105, 174-178. </li></ul><ul><li>Rodgers, (2001). What causes birth order – Intelligence patterns? The admixture hypothesis, revived. American Psychologist, 56(6-7), 505-510. </li></ul><ul><li>Shu, Tien, Lung, & Chang (2000). Norms for the Wisconsin Cart Sorting Test in 6 to 11 year old children in Taiwan. The Clinical Neuropsychologist, 14(3), 275-286. </li></ul><ul><li>Michalski & Shackelford, (2001). Comment: methodology, birth order, intelligence, and personaltiy. American Psychologist, 56(6-7), 520-524. </li></ul><ul><li>Marjoribanks (2001). Sibling correlates of young adults’ status attainment: ethnic group differences. Journal of Psychology, 123(5), 507-516. </li></ul><ul><li>Zajonc, (2001). The family dynamics of intellectual development. American Psychologist, 56(6-7), 490-496. </li></ul>
    31. 31. (bibliography continued) <ul><li>Spiker, Lotspeich, Dimiceli, Szatmari, Myers, & Risch, (2001). Birth order effects on nonverbal IQ scores in autism multiplex families. Journal of Autism and Developmental Disorders, 31(5). </li></ul><ul><li>Hopkins & Dahl (2000). Birth order and hand preference in chimpanzeees: implications pathological models of handedness in humans. Journal of Comparative Psychology, 114(3). </li></ul><ul><li>Bogaert, (1997). Birth order and sexual orientation in women. Behavioral Neuroscience, 111, 1395-1397. </li></ul><ul><li>Blanchard, (2004). Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men. Journal of Theoretical Biology, 230, 173-187. </li></ul>

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