In order to assert that the brain is behavior, one must first take into account the mind's capacity to exert control over/direct the nature of behavior through mental functions such as emoting, thinking, feeling, etc. For example, the mere anticipation of a stressful event can initiate a whole host of physiological changes commaneered by the endocrine system which mimics sympathetic nervous system arousal. If we broaden the parameters of what we consider to be behavior to include mental functions, then in a sense we are asserting that the mind and the brain are one.
In order for this train of thought to be consistent, as mental functions/activity can affect physiology, then the reverse ought to hold as well, that behavior has the potential to influence the mind. Such appears to be the case with regard to facial expressions. Anthropologists and psychologists have widely found that not only are universal facial expressions used to reflect affective states but that the formation of specific facial expressions elicits corresponding affective states. For example, smiling has been found to elicit feelings of happiness. Such an example indicates that the body and the mind are so inextricably linked that the nineteenth century relegation of the automatic, involuntary, unconscious and reflexive aspects of behavior to physiology and the conscious, voluntary and non-rigid behaviors to the mind is no longer a valid disctinction.
Just as our mental states can affect our physiological responses and our behavior affects our mental states, brain structures have been isolated, the activity of which both drives behavior and reflects changes in our behavior. For example, the role of directing the regulation and maintenance of homeostasis within the body appears to lie within the hypothalamus. It works in conjunction with the endocrine system to motivate the initiation of "survival behaviors" such as sleeping, eating, and sexual activity. Two examples of localization of mental functions are the amygdala's role in the regulation of emotion and the mediation of reward and pleasure mechanisms by the ventral tegmental area, the medial forebrain bundle and the limibic system. Even complex mental functions such as learning are reflected in long-term changes in the influence of one neuron on the likelihood that an adjacent neuron will fire if it does. Changes in neural signals generated by the hippocampus can be observed when a rat engages in goal-directed behavior such as walking and exloring its environment.
In a sense, everything that we do, think, and are corresponds to our brains -- their structures and activation. The question then becomes one of if we all share the same basic brain structure, how do individual differences arise in behavior, personality, abilities, etc.? For many years, scientists thought that differences in intelligence were reflected in individual differences in the shape and size of the skull which was assumed to indicate the shape and size of the brain. Stemming from Gall's pseudoscience of phrenology, this tradition claimed that racial differences in skull shape and size relflected differences in mental abilities. While these ideas have been highly censured, and rightly so, this line of brain research focusing on the relation between differences in size and variation in function as expressed in behavior continues today. Researchers are comparing the sizes and shapes of brain structures such as the corpus callosum and the superchiasmatic nucleus in men and women as indication of gender differences in cognitive abilities. Other researchers have focused on differences in the size of INAH-3 of the hypothalamus in male heterosexuals and homosexuals as an indication of a biological basis for differences in sexual orientation. Admittedly, these assertions are highly controversial; however, they all suggest that our behaviors and our interactions with our environment both reflect and are reflected in brain structure and function.
Beyond structure and cell size, shape and density, it has also been found that neurophysiological changes occur in the brain as we interact with our environments and through our experiences in a larger sense as mentioned above. Given such changes, even though we begin life with very similar structures, as they develop, our brains are constantly changing at the neurochemical, structural and cellular levels. And it is in the nature of our interaction with our environments and in our innate ability to adapt to changes in our external and internal evniornments that our individual differences are reflected in not only our overt behavior but also in the complexity of the interconnectedness of our brains.
Very sophisticated treatment of complex subject. Indeed the causal arrows should be regarded as running in both directions in all the cases you describe. And while particular instances may be controversial, the general expectation that individual behavioral differences correspond to individual brain differences is to be expected on the brain=behavior argument. PG