Masculinity has its roots in social constructions and genetics. Therefore while masculinity can mean different things to different cultures, there are often common aspects to its definition across cultures. Sometimes gender scholars will use the phrase "hegemonic masculinity" to distinguish the most dominant form of masculinity from other variants. In the mid-twentieth century United States, for example, John Wayne might embody one form of masculinity, while Albert Einstein might be seen as masculine, but not in the same "hegemonic" fashion.

Most men feel pressured to act masculine. These men feel that they have to prevail in situations that require physical strength and fitness. To appear weak, emotional, or sexually inefficient is a major threat to their self-esteem. To be content, these men must feel that they are decisive and self-assured, and rational. Masculine gender role stress may develop if a man feels that he has acted 'unmanly'. Conversely, acting 'manly' among peers will often result in increased social validation or general competitive advantage.

Cited from Wikipedia

• Real Men Get the Blues - TIME Magazine
• The impact of gender role conflict on multidimensional social support in older men

Below are links to websites that you may find useful:

• The Men's Center
• Masculinity
• APA Journals