Here’s a definition of dimorphism as given in The Encyclopedia of Evolutionary Biology: “Any trait that differs on average between sexes is considered sexually dimorphic, even if the trait distributions overlap considerably between sexes.”1 You will sometimes read claims that a trait is not sexually dimorphic unless it is completely different in the two sexes. That definition has no scientific standing.
For all organisms that reproduce sexually, the biological definition of sex is based on the comparative size of their gametes—the germ cells that can unite to produce offspring. Males are defined as the sex with the smaller gamete and females are defined as the sex with the larger gamete.2 The gametes in humans are sperm and the ovum. The sperm is the smallest cell in the human body and the ovum is one of the largest. In that sense, there’s no question about whether humans come in just two sexes nor about who is which sex.
Physiologically and psychologically, there is more room for variation, but the basics may also be stated simply: Physiologically, human sexuality comes in two forms, male and female, with an extremely small proportion of exceptions. Psychologically, human sexuality forms a continuum from “completely heterosexual” to “completely homosexual,” but proportionately few people self-identify as anything other than heterosexual.
As I set out to review the data, I will state explicitly what should go without saying: Human dignity and human rights are universal, unconnected to sexual identity. I am presenting data on frequency distributions that have no relevance to value judgments. The reason I need to present the data on those frequency distributions here is because of the descriptions of sex differences in the main text. I write in the face of a yawning gap between popular perceptions about gender identity and scientific reality. For example, Gallup polls since 2000 have consistently found that Americans think that about 23 percent of the population is gay or lesbian.3 As you will see, the actual figure is closer to a fifth of that. Hence the need to provide proof that all but a few percent of humans are heterosexual males or heterosexual females.
Here are brief answers to four questions: What proportion of people have biologically ambiguous sexual identity—to use the contemporary term, are intersexuals? What proportion of people self-identify as homosexual or bisexual? What proportion of people acknowledge any same-sex attraction or behavior? What proportion of people self-identify (regardless of their biological sex) as transsexual?
Biological Sexual Identity
Before genetics got into the picture, the definition of biological sex was based on physiological differences in the genitalia, leaving one option for identifying intersex persons: people whose genitalia had some of the characteristics of both males and females, known as hermaphrodites. Once the sex distinction in chromosomes was discovered early in the twentieth century—females have the XX pair and males have the XY pair—cleaner definitions of three types of intersexuals were possible: true hermaphrodites (persons born with both testicular and ovarian tissue), people with the XX chromosome pair but significant components of male sexual anatomy, and people with the XY chromosome pair but with significant components of female sexual anatomy.4
The three conditions that are consensually accepted as evidence that a person is intersex are as follows:
True hermaphrodites. Individuals born with both testicular and ovarian tissue.5 Some of the people with the other two conditions appear to have the characteristics of both male and female genitalia, but they don’t meet the technical definition.
Classic congenital adrenal hyperplasia (CAH) among females, discussed in chapter 5. Classic CAH is the result of prenatal exposure to abnormally high levels of male hormones (androgens). In female infants, CAH usually presents as ambiguous genitalia. In children and adults, it is associated with masculinization of features, facial hair, menstrual problems, and male-typical personality characteristics. In males, sexual ambiguity is rarely involved nor are there usually any symptoms at birth.6 Classic CAH in males is accompanied by abnormally rapid growth in childhood, early signs of puberty, and shorter than expected height as an adult.
Androgen insensitivity syndrome (AIS), also discussed in chapter 5. AIS affects only genetically male (XY) fetuses. The condition impairs masculinization of male genitalia. In its mild form, the external genitalia at birth are those of a normal male; in its extreme form, “complete” AIS (CAIS), the external genitalia are those of a normal female.
The table below summarizes incidence of the three conditions. The note gives the source and additional information about the numbers.[7]
True hermaphrodites
Percent: 0.0012
Classic congenital adrenal hyperplasia (CAH)
Percent: 0.0077
Androgen insensitivity syndrome (partial or complete)
Percent: 0.0084
Total
Percent: 0.0173
Limited to the three conditions that are consensually accepted as evidence of an intersex individual (including all levels of AIS), the proportion of intersex persons is 0.0173 percent—less than two-hundredths of a percentage point, or about one in 5,800 persons. That number is slightly inflated because the estimate for CAH includes males as well as females.
In 2000, this core definition was broadened by a team of scholars (first author was Melanie Blackless).8 Their criteria for a “typical” male or female were, in the authors’ own words, “exacting”:
We define the typical male as someone with an XY chromosomal composition, and testes located within the scrotal sac. The testes produce sperm which, via the vas deferens, may be transported to the urethra and ejaculated outside the body. Penis length at birth ranges from 2.5 to 4.5 cm; an idealized penis has a completely enclosed urethra which opens at the tip of the glans. During fetal development, the testes produce the Mullerian inhibiting factor, testosterone, and dihydrotestosterone, while juvenile testicular activity ensures a masculinizing puberty. The typical female has two X chromosomes, functional ovaries which ensure a feminizing puberty, oviducts connecting to a uterus, cervix and vaginal canal, inner and outer vaginal lips, and a clitoris, which at birth ranges in size from 0.20 to 0.85 cm.9
The authors subsequently referred to this as “the Platonic ideal of sexual dimorphism.” The additional conditions that fall short of the Platonic ideal of sexual dimorphism under that definition fall into three categories (plus a small one of “other”): some chromosomal arrangement other than XX for women and XY for males, vaginal agenesis, and late-onset CAH. The difficulty with counting these conditions as evidence of an intersex individual is that they rarely involve sexual ambiguity. Affected males almost always think of themselves as male; affected females almost always think of themselves as female.
Males with non-XY chromosomes. This includes males with Klinefelter syndrome (an XXY chromosomal makeup) and XXY, XO, XYY, and XXYY variants. These conditions sometimes cause infertility and other physiological and cognitive problems in males, but none of them are associated with confusion in sexual identity. They are biologically males in every clinical sense of the term.
Females with non-XX chromosomes. This includes females with either Turner syndrome or an XXX genotype (Triple X). Women with Turner syndrome are partly or completely missing one of their X chromosomes. The condition brings with it a variety of problems, including infertility, short stature, and short life expectancy. The literature has found that women with Turner syndrome have a tendency to be hyperfeminine.10 Women with the XXX configuration sometimes have developmental cognitive difficulties, but once again function normally as women without an association with sexual ambiguity.11
Vaginal agenesis. In its simplest form, vaginal agenesis consists of fibrous tissue that displaces a portion of normal vaginal tissue. Correcting it surgically is a straightforward procedure that has been likened to correcting a cleft palate.12 Vaginal agenesis does have one serious and common characteristic however: The uterus is absent or underdeveloped. But this makes a female no less female than infertility makes a male less male. Women
with vaginal agenesis have the XX genotype and normal hormonal exposure for females both in utero and after birth.13
Late-onset CAH. That leaves us with late-onset CAH, which in the Blackless study is assigned an incidence rate of 1.5 percent. “Late-onset CAH” refers to a mild form of CAH that appears in childhood or near puberty.
For females and male infants alike, the genitalia appear normal at birth and correspond to the normal chromosomal makeup: All the XY individuals have penises and testicles and all the XX individuals have vaginas and ovaries. For females, menstrual irregularities account for over half of the presenting signs for diagnosis among adolescents.14 Other symptoms can include rapid growth in childhood but shorter than expected eventual height, early signs of puberty, and acne, but the average woman with late-onset CAH does not present until about age 24.15 For adult women, presenting symptoms may include enlargement of the clitoris, excess facial or body hair, and, for about 10–15 percent of cases, fertility problems.16
Since late-onset CAH is an autosomal recessive disease; it presumably occurs equally in men and women, but far fewer men than women are identified in the technical literature. The reason, as the authors of a 2017 systematic review of the literature noted, is that “the great majority of male patients are asymptomatic and most are identified during genetic screening carried out for purposes of genetic counseling.”17 In adolescent males, the symptoms are most likely to be early appearance of pubic hair, rapid growth during childhood but shorter-than-average eventual height, and early male pattern baldness.18
This is not to claim that late-onset CAH never has symptoms that introduce sexual ambiguity in the sense that classic CAH does. Rather, the evidence is clear that such cases are extremely rare among females diagnosed with late-onset CAH and close to zero among males diagnosed with late-onset CAH. Here are the estimated incidence rates for the Blackless study’s additions to the core intersex conditions. See the note for sources and details.[19]
Turner syndrome
Percent: 0.0369
Klinefelter syndrome
Percent: 0.0922
Other non-XY chromosomes for males, non-XX for females
Percent: 0.0639
Vaginal agenesis
Percent: 0.0169
Late-onset CAH in both men and women
Percent: 1.5000
Unspecified, cause unknown
Percent: 0.0009
Total
Percent: 1.7108
Perhaps the most important point about all the above departures from a “Platonic ideal of sexual dimorphism” is that none of the sources I have listed discuss sexual ambiguity as among the presenting symptoms. Is it appropriate to define these people as “intersexuals”?
I leave it as a question. The answer doesn’t matter in any practical sense for establishing that humans consist of two sexes with a small number of exceptions. Depending on what you think of the additional departures from the Platonic ideal—and especially what you think of late-onset CAH—either 98.3 percent or 99.8 percent of the population are unambiguously male or female in the biological sense. Either figure makes my point.
I should add, however, that the answer does matter from the clinician’s point of view. The total proportion of people considered intersexual in the Blackless study is 1.728 percent, which is almost exactly 100 times the total proportion of people—0.0173 percent—considered intersexual based on the three core categories of intersexuality. One clinician (also a psychologist and physician), Leonard Sax, observed that the total for the three core categories “suggests that there are currently [2002] about 50,000 true intersexuals living in the United States. These individuals are of course entitled to the same expert care and consideration that all patients deserve. Nothing is gained, however, by pretending that there are 5,000,000 such individuals.”20
Self-Identified Sexual Orientation
I begin with the ways in which people describe their sexual orientation independently of measures of sexual behavior or attraction.
In 2011, Gary Gates of the Williams Institute published a synthesis of the major studies up to that time that had asked adults to identify their orientation as heterosexual, gay, lesbian, or bisexual. The Gates study combined the results from five American surveys conducted from 2004 through 2009 to reach overall estimates. For males, the results were estimates of 2.2 percent gay and 1.4 percent bisexual. For females, the numbers were 1.1 percent lesbian and 2.2 percent bisexual. Overall, Gates put self-identified lesbian, gay, and bisexual (LGB) individuals at 3.5 percent of the population.21
READING INTO THE STATE OF KNOWLEDGE ABOUT SEXUAL IDENTITY
Standing apart from the rhetoric about gender fluidity and the existence of multiple genders is a body of empirical work that still includes many controversies, but ones that are being argued out in the technical literature the old-fashioned way, with actual data. For an overview of where things stand on the major issues, including reviews of the literature on definitions, measurement issues, prevalence, sex differences in expression of sexual orientation, sex differences in category-specific sexual arousal, sexual fluidity, developmental and psychological correlates of sexual orientation, bisexuality, and the environmental and genetic causes of sexual orientation, I recommend a 56-page article, “Sexual Orientation, Controversy, and Science,” published in 2016 by a team of the field’s leading scholars of varying perspectives (Michael Bailey, Paul Vasey, Lisa Diamond, Marc Breedlove, Eric Vilain, and Marc Epprecht).22
One of those sources was the General Social Survey, which had asked the question for the first time in 2008 and has continued to ask it through 2016. We also have subsequent data from an annual poll question about sexual identity that Gallup introduced in 2012 and estimates from the National Health Interview Survey (NHIS). The latest published percentages as I write are as follows:23
Gallup Daily Tracking Poll
Year: 2017
Sample: 340,604
Total LGB: 4.5%
General Social Survey
Year: 2016
Sample: 1,743
Total LGB: 5.9%
Nat’l Health Interview Survey
Year: 2015
Sample: 103,789
Total LGB: 2.4%
Both the Gallup data and the General Social Survey show steady though small increases over time. An unweighted average of the three is 4.3 percent; an average weighted by sample size is 4.0 percent.
These estimates of the LGB population may strike you as absurdly low (recall that in polls, Americans estimate that about 23 percent of the population is gay or lesbian). But they are actually higher than the estimates that have been found in the other Western countries that have reported on self-identified sexual orientation for nationally representative samples.[24] Here are the results of the most recent major studies conducted outside America that I have been able to find:
Norwegian Living Conditions Survey (2010)
Gay/lesbian: 0.7%
Bisexual: 0.5%
Total: 1.2%
UK Integrated Household Survey (2016)
Gay/lesbian: 1.2%
Bisexual: 0.8%
Total: 2.0%
Canadian Community Health Survey (2014)
Gay/lesbian: 1.7%
Bisexual: 1.3%
Total: 3.0%
Australian Study of Health and Relationships (2014)
Gay/lesbian: 1.6%
Bisexual: 1.7%
Total: 3.3%
New Zealand Attitudes and Values Study (2013–14)
Gay/lesbian: 2.6%
Bisexual: 1.8%
Total: 4.4%
There may well be some degree of undercount. But there has been a revolution in openness about homosexuality in all of these countries that is now several decades old. Homosexuality has even acquired cachet among some circles in all of these countries. The answers to all of these surveys were anonymous. It is hard to come up with a scenario whereby all of the reported results are radical undercounts of authentic proportions of se
lf-identified gays, lesbians, and bisexuals.
Prevalence of Same-Sex Attraction or Behavior
Self-identified sexual orientation is an undercount in another sense, however. How does one characterize a person who had a few homosexual experiences as a teenager and not thereafter? A person who was once sexually attracted to another of the same sex but didn’t act on it? A person who has been sexually attracted to others of the same sex several times but never acted on it? A person who has had sexual relations with both sexes, but has a decided preference for one of them? For that matter, how does one classify people who have never felt same-sex attraction but have occasional curiosity about what it’s like? That must include just about everyone.
In the same 2011 study I have been referencing, Gates included a synthesis of major studies that had tried to assess the proportion of people who have ever experienced any homosexual attraction.25 Gates reported an American incidence of 11.0 percent, much higher than an Australian incidence of 6.5 percent and a Norwegian incidence of 1.8 percent. But only a minority of those who have experienced any homosexual attraction are equally attracted to both sexes—in the American study that produced the 11.0 percent figure, only 3.3 percent of all respondents said they were equally attracted to both sexes.26
Gates also reported the proportion of people who have ever engaged (even if just once) in same-sex sexual behavior. The answer for Americans was about 8 percent (two studies came up with incidence rates of 8.8 percent and 7.5 percent respectively) and 6.9 percent for Australians.27
One common way of trying to capture the continuum from “completely heterosexual” to “completely homosexual” is to ask respondents to put themselves on a five-point scale with the options of “heterosexual,” “mostly heterosexual,” “bisexual,” “mostly gay/lesbian,” or “gay/lesbian,” or else on the similar six-point Kinsey scale. In 2013, Ritch Savin-Williams and Zhana Vrangalova compiled a systematic literature review on people who answered “mostly heterosexual.”
Human Diversity Page 41