Certianly the brain plays a huge (if not the entire role) in behavior, however, I feel that reducing behavior to processing of inputs by neurons and neuronal tissue, devalues the complexity of human behavior. Human behavior is a complex product of not only stimuli reception and processing, but also of choice which is based on a multidimensional process of integrating past experiences, current perceptions, feelings and emotions with anticipation of how this choice or event might shape or influence future events. Let's take the behavior of choosing one's mate. The initial attraction stage might be influcenced by physical appearance (sensory reception and perception of features), ones odor, and ones preference for leisure activities. However, the choice will also potentially be influenced by their previous experience with similar or different individuals (past experience), the current situation and opinions from people that one respects. These perceptions and feelings are also influenced by the individuals expectations for the future: will this person enjoy children, will they continue to enjoy the same things in life, etc. Although all these factors influencing the choice are processed through the nervous system, they are influenced by an infinate number of variable along the way. Hence, the complex behaviors of humans are not only a result of stimuli and response, but also the choices, feelings, emotions and probably many other factors that I forgetting to mention. The point is , human behavior is complex, and I fear that atributing it the nervous system alone reduces it's complexity and richness untill we can understand "how" these processes occur. How does one make a choice? What factor's influence this choice? How is this process accomplished in terms of neuronal processing and brain function? Until we have the answers to these questions I feel it is a leap of faith to assume the all behavior is a result of nervous system processing. Although science has been able to demonstrate this in less complex animals, such as in the chirping of crickets, we have not been able to provide the physical evidence for this in humans. Neurotransmitters, neuropeptides and other chemical processing has provided evidence that processing of information is complex with not only several different types of neurochemicals, but also perhaps an infinite number of receptor types. What guides this compexity in terms of it's development and functional significance? Why do some individuals who develop pathological behaviors that respond to medications that are designed to mimick or replenish unbalanced or absent neurochemicals and others do not? Why do some individuals recover from CNS injury better than others? Until we have more substantial answers to these questions, we can only continue to seek answers through nervous system functioning; but I suggest we keep our minds open to other paradigms that might help us explain, and therefore develop treatments for disruptions in behavior.
Yes, by all means, let's keep mind (brain?) open. At the same time, neither individual differences nor complexity per se seem in principle difficult to account for in terms of the brain. There's LOTS of things there, so lots of potential for both (as well as for feelings, emotions, choices). PG