The term "Manspreading" is a relatively recently coined colloquial for what, in more technical nonverbal terminology, is referred to as the "Seated Crotch Display". Manspreading usually describes, but is not limited to, relatively crowded public setting such as a subway, bus or airplane.
The seated crotch display is seen much more commonly in males for several reasons. The Shape of the human pelvis tends to favor a mildly larger angle of the thighs in men vs. women - although this is in no way explains the more extreme manspreading angles very commonly exampled by men. External genitalia is another reason for larger seated thigh angles with men - but again, only a mildly larger amount of thigh separation is necessary to accommodate this difference in anatomy.
When for example, several heterosexual men - who happen to be co-workers are seated around a table (if, say they're at lunch and exists good rapport and even friendship) - the vast majority of time, men will sit exhibiting a considerably wide-angle crotch display. This is very similar to a sub-troop of young, friendly chimpanzees or gorillas beating their chests while displaying dominance yet in the midst of playing. In this context, a seated crotch display is essentially a default normal setting in most men.
Intriguingly, during a business meeting, men will typically sit in a manspreading fashion in a (subconscious) effort to assert dominance - yet they'll be completely oblivious to the fact they're behaving offensively to their women co-workers.
Men (much more commonly than women) will also subconsciously sit in a seated crotch display when they are sexually attracted to someone in the immediate vicinity - or when they are feeling sexual while on camera, on stage, etc. Thus in these contexts, a seated crotch display serves as a subconscious signal of sexual readiness and for a potential partner to reciprocate advances - e.g., "I'm giving you a green light, please proceed in kind."
Women will tend to sit with their legs closer together because they have been strongly socialized and taught very specifically to do so - with at least one objective being the avoidance or at least minimization of any unwanted advances from men. Also, because the vast majority of human cultures are highly patriarchal - most societies consider the act of a seated crotch display in a girl or woman to be very "unladylike". Unfortunately, as one of the participants in this video said (beginning at 2:22), "Women are expected to be small, quiet and almost invisible - whereas men can be these boisterous slobs and it's all good." Such misogynistic beliefs certainly contribute to the gender disparity of this particular body language display.
High vs. low confidence levels (and Alpha vs. Beta personalities) also contribute to manspreading (or not). When we feel less confident we all tend to sit with our thighs a smaller angle and when we're feeling more confident this angle tends to increase. This dynamic is more obvious with men because of the extremes in leg position during times of confidence is high. Of course, there are many other nonverbal displays signaling relative confidence levels - this is but one.
Autism Spectrum Disorders occur at a significantly higher rate in men - thus a portion of those who manspread have this as a reason for their diminished sensitivity to the importance of others' personal (intimate) space.
A significant share of those who sit in a manspreading fashion can be classified as having Narcissistic Personality Disorder. They have relatively low (or perhaps nonexistent) empathy quotients - and therefore not only just don't care about the people sitting next to them, but they want to deliberately invade another person's personal and intimate spaces.
Summary: While Manspreading (aka Seated Crotch Display) is a behavior with many contributing factors - we would all be well-advised to dial up our empathy when seated in crowded areas.
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