The Real Sex Binary
In the social sciences, many scholars define sex as a collection of traits—X/Y
chromosomes, gonads, hormones, and genitals—that cluster together in most people but may
also occur in rare atypical combinations (e.g., Blakemore et al., 2009; Fausto-Sterling, 2012;
Helgeson, 2016; Joel, 2012). This definition is the basis for the widely repeated claim that up to
2% of live births are intersex (Blackless et al., 2000; see e.g., Hyde et al., 2019). In fact, the 2%
figure is a gross overestimate. Blackless et al. (2000) defined intersex very broadly as individuals
who deviate from the “Platonic ideal” of sex dimorphism; accordingly, they included several
conditions (e.g., Klinefelter syndrome, vaginal agenesis, congenital adrenal hyperplasia) that
affect sexual development but can be classified as “intersex” only in a very loose sense (Sax,
2002). If one restricts the term to conditions that involve a discordance between chromosomal
and phenotypic sex, or a phenotype that cannot be classified unambiguously as either male or
female, the frequency of intersex is almost certainly less than 0.02% (Sax, 2002; see also Hull,
2003).
On a deeper level, the “patchwork” definition of sex used in the social sciences is purely
descriptive and lacks a functional rationale. This contrasts sharply with how the sexes are
defined in biology. From a biological standpoint, what distinguishes the males and females of a
species is the size of their gametes: males produce small gametes (e.g., sperm), females produce
large gametes (e.g., eggs; Kodric-Brown & Brown, 1987).4 Dimorphism in gamete size or
anisogamy is the dominant pattern in multicellular organisms, including animals. The evolution
of two gamete types with different sizes and roles in fertilization is the predictable consequence
of selection to maximize the efficiency of fertilization (Lehtonen & Kokko, 2011; Lehtonen &
Parker, 2014). In turn, anisogamy set the stage for sexual selection (i.e., selection via mating
competition and mate choice), with predictable consequences for the evolution of sexually
differentiated traits in morphology, development, and behavior (Janicke et al., 2016; Lehtonen et
al., 2016; Schärer et al., 2012). Of course, the existence of two distinct sexes does not mean that
sex-related traits must also have binary, sharply bimodal distributions. The sex binary is
perfectly compatible with large amounts of within-sex variation in anatomy, physiology, and
behavior. In fact, sexual selection often amplifies individual variability in sex-related traits
(typically more strongly in males), and can favor the evolution of multiple alternative phenotypes
within each sex (see Del Giudice et al., 2018b; Taborsky & Brockmann, 2010).
To be clear, the biological definition of sex is not just one option among many equally
valid alternatives; the very existence of differentiated males and females in a species depends on
the existence of two gamete types. Chromosomes and hormones participate in the mechanics of
sex determination and sexual differentiation, but do not play the same foundational role. The sex
binary, then, is not a fiction but a basic biological fact: even if a given individual may fail to
produce viable gametes, there are only two gamete types with no meaningful intermediate forms
(Lehtonen & Parker, 2014; see also Cretella et al., 2019). This dichotomy is not statistical but
functional, and hence is not challenged by the existence of intersex conditions (regardless of
their frequency), nonbinary gender identities, and other seeming exceptions.
