|The brain is the most complex organ.|
The Brain's Structure Affects Behavior
This is the nature part of the feedback loop. The structure of the brain can affect a person's personality and behaviors. For example, a person's behavior can change after his brain is damaged, as in the case of Phineas Gage--who provides the prime example in most Introductory Psychology Books. Phineas Gage had a tamping iron shot through his head and it damaged his frontal lobe--which got rid of his inhibitions. He was said to have changed his personality from a normal, pretty upstanding guy to a complete jerk and drunkard. The fact that the brain's structure affects behavior and personality implies that these things are genetic. Truly they are. In fact, even the way we use language has genetic components to it, as evidenced by the KE family.
This follows through to dogs. Dogs have been bred selectively to display different instincts and behaviors and temperaments depending on what job they are being designed for. For example, Beagles sniff the ground and Greyhounds chase. Likewise, for the instincts to be useful for the tasks that they are bred for, the dogs must have some training. This leads to the next point.
Environment & Behavior Affects the Brain's Structure
This is the nurture part of the feedback loop. What is meant by this is that the environment that you live in and the behaviors that you engage in change the structure of your brain. This is what neuroplasticity is all about. Basically, practice makes perfect. A good example of this is the differences in the brains of monks that have meditated for thousands of hours vs. those of people that have not. Because the monks were always meditating, their brains showed more activity in compassion at the sight of suffering and movement toward compassion, indicating that not only did they want to help, they were willing to physically help as well. In fact, the reason why people develop habits is because the act of doing the habit makes it more likely in the future for them to the action again. So essentially, practice makes perfect because practice literally changes the brain to make it easier, in the future, to do the behaviors you practice.
This also goes a step further. There is also the field of epigenetic research. Epigenetics is the changing of a gene's expression in response to the environment and I will go further into this in future post.