Biological sex

Jump to navigation Jump to search


Biological sex is a scientific term which is widely misused.

Key Points

  • "Biology" is not synonymous with reproduction.
  • "Biological sex" has many different characteristics, and they do not always align neatly into "male" and "female" boxes.
  • Behavior is partly biological too.

See Issuepedia for additional general information on this topic, and gender/101 in HTYP for more on how it relates to gender.


Much as with gender, biological sex is a multidimensional continuum – there are many aspects to it, and generally they do not fit neatly into a purely binary categorization of "male" or "female":

  • chromosomes (XX, XY, or any of many other types)
  • genital configuration (inny, outy, or intersex)
  • skin tone (adult females tend to have softer, lighter skin with a layer of subcutaneous fat)
  • skin padding (adult females typically have additional fatty deposits in certain areas, giving them a more "curvy" shape)
  • body & facial hair (adult males tend to have more hair everywhere except the top of the head, especially on the face and chest)
  • sex hormone balance (mix of androgen, estrogen, and progestens)
  • back of the skull (adult males have a bump which children and adult females lack)
  • musculature (adult males tend to have greater upper-body strength)
  • height (adult females tend to be a few inches shorter than adult males)
  • reproductive
    • whether you contribute an egg or sperm
    • whether your body hosts the embryo
  • structure of the brain

The last item is of particular note as it heavily influences behavior, which is often relegated to a separate category from "biology".


Anti-trans advocates frequently use "biological sex" as if it were monolithic and unambiguously binary. They also frequently seem to imply that it is synonymous with what genitalia you appear to have at birth.

It is neither of these, as numerous uncontroversial counterexamples have shown:

  • monolithic:
    • You can have XY chromosomes but be born with a vagina and "female biology" in all other respects (androgen insensitivity syndrome).
    • You can have XY chromosomes but be born appearing with a vagina (which turns into a penis at adolescence), and identify as male (Guevedoces).
    • You can identify as and appear clearly female, but be exceptionally strong, or have facial hair, or be very tall (or the reverse).
    • You can appear male facially but in all other ways be female (and vice-versa).
    • (There are many more exceptions.)
  • unambiguously binary:
    • You can have ambiguous genitalia at birth (intersex).
    • You can have neither XX nor XY, but some other combination.
    • You can be a genetic mosaic, e.g. with functional female reproductive organs but a mostly XY body.
    • (There are many more exceptions.)