I believe one of the most important concepts or debates ever to surface throughout the history of psychology is the nature-nurture debate. The debate is about whether a person's behavior and mental state are influenced by society or their family, the way they are nurtured, or if they are a direct result of their genetics, their nature.
Although recently many scientists in the social sciences field have reached an agreement that both nature and nurture are together relevant and important in shaping a person's psyche, there are still debates out there that one is more relevant than the other, or for some aspects of the psyche, one is completely irrelevant compared to the other.
For example, I found an article while surfing the internet one day about homosexuality and how, at one point, it was thought by the American Psychological Association to be a mental disorder. There were various experiments done by various scientists to figure out if homosexuality was influenced by nature or by nurture. It was clear that scientists believed that homosexuality was either nature or nurture; not both. That article can be found here: http://allpsych.com/journal/homosexuality.html
The nature-nurture debate is directly relevant to me because my younger sister is adopted. I have always been interested to see in which ways she has grown up different from me, and in which ways she is the same. She is of the same ethnic background, and was adopted when she was three, but she is still different from me in various ways such as height, body type, metabolism, and interests. She is the same in ways such as speech style and, to some extent, decision-making.
I guess what I still want to know is why the nature-nurture debate still exists to the extent that it does when it has been proven that both nature and nurture affect everything that shapes the human psyche.