In my survey, I also asked participants to rank, on a scale of 1 to 5, how much their family, school, media, and extracurriculars shaped their views of what it means to be a man.
Here are the results. Family was, on average, most influential, followed by the media. School and extracurriculars were still influential, just not as strongly.
As you can see, social factors strongly influence many men's views of masculinity.
The stereotypes about men that TV shows and movies portray, as well as the beliefs parents and teachers have about what it means to be a man, can all impact how a young man develops. The way men act and prove themselves to be manly is clearly not a natural instinct, it is largely shaped by social factors.