At what age do children develop a sense of gender identity?

Question

To make things clear, I want to state that I have no doubt about whether people are transgender and what that means. I simply am asking about what age children are able to comprehend their personal situation.

The family of an adult who recently came out as transgender sent me the PDF by PFLAG titled Our Trans Loved Ones: Questions and Answers for Parents, Families, and Friends of People Who Are Transgender and Gender Expansive.

As the parent of two young toddlers, I found it interesting that it states that children with gender dysphoria are aware of it as young as age two. See page 26-7:

"These children.... can start telling us as early as the second year of life, maybe even with their first toddler sentence..."

I related this to my husband, who expressed doubt that a two-year old would be able to understand that and felt it would happen more around age six. The rest of the PDF doesn't help with specifics much, as pages 17-8 just state,

"[O]ur sense of self (including gender) becomes apparent at a very young age; for most this is between the ages of two and four years old, and this awareness remains stable over time."

The document just treats this as established fact.

So where are the studies about this? How do we know that children understand not just that other people can be a boy or a girl, but that they themselves are one and that it will stay "stable over time"? Or, even better, are there studies that show that these children who tell their parents "with their first toddler sentence, 'Me not boy. Me girl.'" overwhelmingly identify as transgender (or gender expansive) at any point after they can hold an intelligent conversation? I mean, for months my 3.5-year-old has been correcting me and telling me that he's "not a little boy" but "a truck." I don't think he's going to identify as nonhuman when he grows up (but I could be wrong, and I would love him just the same). We want data, not anecdotes, even though I know hard data are tough to come by with young children.

Some related research was linked to from an article here: When do children develop their gender identity? but that article is more interested in studies that show children have a more fluid concept of gender and it only becomes rigid later on, which is in some ways the opposite of what the PFLAG document claims.

Answer 1

According to this review of studies, children develop a sense of gender quite early and can relate it to themselves early as well:

A related question is, when do children recognize their own sex? Infants as young as three to four months of age distinguish between categories of female and male faces [...] By 10 months, infants are able to form stereotypic associations between faces of women and men and gender-typed objects [...]

Because of the difficulties associated with testing infants, it has been challenging to determine when children first recognize their own or others' sex. [...]

In another non-verbal testing situation, 24- and 30-month old children knew the gender groups to which they and others belonged (Stennes et al. 2005). Similarly, most 24- and 28-month-old children select the correct picture in response to gender labels provided by an experimenter [...]

Taken together, these studies suggest that most children develop the ability to label gender groups and to use gender labels in their speech between 18 and 24 months. [...] These findings are consistent with research suggesting that children develop awareness of their own “self ” at roughly 18 months _{Martin & Ruble: Patterns of Gender Development}

Regarding the stability of this assessment, that is a bit more difficult. Regarding trans children specifically, you might be interested in this question: Do 80% of gender-dysphoric children grow up deciding not to change their gender?.

The study linked above also looked at the stability of gender typing - not gender identity though. It says that there are few studies with different results on the question, and concludes that:

Over greater numbers of data points and amounts of time, however, relative consistency of individual children can be seen (e.g., Martin & Fabes 2001). [...] Indeed, when stable, reliable measures are used, coherence across indices is often observed (e.g., Martin & Fabes 2001, Serbin et al. 1993). [...] Nevertheless, we also urge caution: It would be unreasonable to conclude that gender typing is strong and stable throughout life, because the database is limited in a number of ways.