Traditional Explanation -
A new emerging area of study is called Epigenetics. We know that environment plays a crucial role in development of a living being. Don’t give a tree water or sun, it probably won’t grow. Smoking (putting carcinogens into the body) over time can cause cardiovascular disease and cancer. Cancer is caused from mutated cellular DNA, that causes cells to not function for their intended purpose. These mutated cells can then grow and cause masses and spread. The carcinogens are an environmental influence that causes health problems. So providing proper environmental stimulus is important to development. When certain criteria is from a stimulus are met, your genes can turn on or off. Allowing the body to adapt best to the environment around it.
Epigenetics and Neurotransmitters -