There was a recent thread about questions you've always wanted to ask the opposite sex. I found it very interesting, mostly lighthearted/tongue in cheek, no expectation that one was talking for all members of a sex but there was a lot of engagement from both sides, no insisting that one way was right and no bollocks about biology. Of course various posters may have had their own beliefs about where certain behaviours/mindsets had come from, fair enough, it wasn't brought onto the thread.
Something that really surprised me and I'd never really thought about before was that several men said that men in general are on constant alert for signs of violence from other men, mentally sizing each other up to judge whether they are a threat, even in an everyday context like work or among strangers the thought is always there and it's a part of how social hierarchies are enforced. It came up in a discussion about why men don't often challenge sexism.
That made me think several things. Firstly that men must learn this from a fairly young age, that this is just how the world works. Smacking children is not common any more, but when it was it wasn't that parents had to do it all the time, the threat would largely be enough. In lieu of smacking, a lot of parenting still involves using the adult's strength to overpower the child - the ultimate message being "it doesn't matter what you do or say, I am stronger than you and that means I will always win". Playground or sibling squabbles happen and often get physical, and yes are dealt with but I think that breaking up the fight, forced apologies, time out etc do nothing for the actual changes which have happened within the hierarchy, which the children involved will be aware of. If you look at all the messages given in children's media and with toys etc, girls are all about friendship, love, caring/nurturing and being likeable (often by being pretty but in other ways too), and boys are given messages about perseverance, loyalty (not friendship), competition (being the fastest, strongest, best) and the clincher, good beating evil, overwhelmingly in a physical fight.
So then secondly having understood that this is the case, as boys and young men they have several opportunities to try this out. School playground as mentioned, football/general pub violence as Buffy said, the competitive nature of the sports boys are encouraged into, play, stories they read/watch, the idea that it's acceptable, even noble, to assault a man who has stolen your girlfriend or hurt a close female friend/relative. Just like men usually don't see the pressure on women to look a certain way despite being exposed to exactly the same stimulus, or seem surprised when it's pointed out that magazines etc encourage this insecurity I think we (women) largely miss this angle as it's not aimed directly at us. But for men, the idea of violence and dominance is so ingrained by adulthood that it is normal and so it always prevails as a back up plan of sorts for dealing with conflict. Hence, men who don't learn other skills of conflict management are more likely to fall back on violence (or non violent physical force, which may be an oxymoron). But it's not an innate thing, the signs are always there.
Lastly the cycle is perpetuated, because it seems so harmless. Just like we say to little girls, without thinking, "Oh your dress is so pretty!" we say to boys "Wow, you are so strong!" and talk merrily about beating the bad guys, as though the bad guys were objectively bad and didn't just stand for somebody that you disagree with.
It's so prevalent, I have a 6yo DS who is obsessed with being the fastest, who loves superheroes, talks often about punching and killing and it's just impossible to counteract these messages when they come so thick and fast.