And this is text from the Cass Report
Differences in gender role behaviours are
apparent in pre-school, when children start to
show gender stereotyped behaviour in their
play. Around this time, they seek to play with
same-sex peers.
Toy choice has been extensively studied.
Researchers classify toys into those that are
typically preferred by boys (for example, cars
and trucks) and those that are typically preferred
by girls (for example, dolls). A systematic review
(Davis & Hines, 2020) demonstrated that these
differences in toy choice are very large.
Like biological characteristics such as
height, there is a large overlap in gender role
behaviours. This variability in gender role
expression exists from an early age (some
girls exhibit behaviours that are traditionally
perceived as more masculine, and some boys
exhibit behaviours that are perceived as more
feminine).
A common assumption is that toy choice
and other gender role behaviours are solely
a result of social influences; for example, that
boys will only be given trucks and girls will
only be given dolls to play with. Although this
is partially true, there is evidence for prenatal
and postnatal hormonal influence on these
behaviours.
Sex differences in the brain emerge in
the second half of pregnancy. There is strong
evidence from animal studies that these
changes are driven by the presence or absence
of testosterone and have a long-term effect on
sex-typed development (Bakker, 2014).
All three of the human characteristics
that show particularly large sex differences
(childhood sex-typed play, sexual orientation
and gender identity) have been found to relate
to early testosterone exposure.
Sex-typed play has been studied more
extensively than any other human behaviour
in this context, and at least 10 independent
research groups have reported a link to prenatal
testosterone exposure (Hines, 2015)