Do you think that masculinity and femininity are social constructs, or are they biologically/genetically determined?

(In other words, do boys/men act masculine and girls act feminine because of what they are taught and because of the models they see, or do boys act like boys and girls act like girls because of their genetic/biological makeup?)

I want to make an argument for each side and be able to defend my answers during discussion. I want to get opinions of what professional think about this topic. Please provide the URLs of the web pages (and/or references of books) if you use any. This doesn't need to be long. Maybe a paragraph or 2. Unless you have a lot to say about it..