Based on the fact that human beings are basically highly advanced and complex animals (homo sapiens), the three main biological bases that are used to explain our psychology are based on primary needs:
1. Self-preservation/rationale for self-preservation- the need to survive, stay alive, and be protected from harm. It refers also to surviving financial, psychological, and physical adversities such as disease, bankruptcy and depression. The need for self-preservation is mainly to survive to produce offspring and defend them throughout life. The rationale for self-preservation is primarily to reproduce and build a bloodline. This is also part of the "human animal."
2. Sex- as part of the need for reproduction, the human animal is able to find egocentric and generic satisfaction from the process of reproduction. The need to reproduce and find ways to do it is part of our behavior as well. If there is a psychological malfunction in this process the result is deviance.
3. Greed- the want of acquiring goods for the sake of protection, need for survival, and for the general feeling of power over others drives greed. This is viewed from the point of view of the hunter and gatherer more so than of the malicious greedy people, say, from Wall street or the like.
Therefore these strong biological and inherent traits are the basis of human behavior because they take their strength from the most used mechanisms, with are self-defense, reproduction, and the acquisition of all that is needed to survive within adversity.