Its socialisation, boys aren’t aggressive, girls aren’t manipulative, but a large number are encouraged to act in that way so it becomes part of their normal behaviour. Even fairly young babies are treated differently if they are perceived to be boys/girls.
It isn’t just boys we are failing by only allowing them to show aggressive ‘manly’ emotions, we’re failing girls by only allowing them to be ‘feminine’ and ‘gentle’.
Boys are split into two groups ‘boys with be boys’ which gives praise for negative behaviour, or ‘mummys boys’ which receives negative attention for what is actually positive behaviour.
Girls are labelled as ‘tom boys’ for completely normal behaviour and they’re discouraged from showing frustration, anger etc.
Look at careers, girls are likely to be encouraged to be nurses, boys doctors. Society often treat people poorly for their job choice, look at the threads on here where people have said they wouldn’t want/use a male childminder, nursery nurse etc. Girls who want to be plumbers, electricians etc are encouraged not to because delicate little girls couldn’t possibly work with ‘male banter’. Being encouraged into a career you will enjoy needs to be normalised, not a career that suits your genitals.
I have a rugbytots franchise and we have girls making around 1/3 of the classes. Girls and boys have the same behaviour expectations, girls and boys are also treated the same when injured, none of this boys need to man up or girls need to be treated like delicate princesses rubbish. This however isn’t popular with some parents, particularly parents of boys and we have had to remind parents that crying, being upset, worried etc are normal and healthy emotions for all children, not just girls.
My siblings and I were brought up with very strict gender roles, boys weren’t ever encouraged to help in the home etc, girls weren’t pushed academically etc. The only thing that differed is that I’m from a culture where boys being sad, crying, worried etc is not seen as a negative, and girls being strong, angry etc also isn’t seen as a negative. Male family members would openly cry, hug, kiss etc. Female family members would openly express anger, frustration etc. Admittedly it is the only thing they got right.
My son is sometimes mistaken for a girl as at the moment we are following the kesh, I often hear a parent telling their son to be careful of that little girl, but when my son shouts that he is a boy the play carefully message is withdrawn. Girls don’t need to be treated with kid gloves, but at the same time boys shouldn’t be treated like punch bags. As it stands he hates rough play or people being generally boistrous, so focuses on play equipment that can only be used by one person at a time.
We all have bias, but as adults it is something we are able to put aside to make sure we’re treating people fairly.