biological causes of alcohol addiction include each person's unique physiology and genetics. People differ in the degree to which they like or dislike a particular addictive substance or activity. Some people may enjoy a substance or activity so much that it becomes very tempting and difficult to resist. Another person would not experience this difficulty because they do not experience a similar enjoyment. Likewise, the ability to temper impulsive desires with rational thought is a brain function that varies among different people. Some people may have a deficiency in their capacity to resist certain types of impulses. Thus, these folks would be at greater risk for developing an addiction because of their genetic vulnerability. Oddly enough, even normal human brain functioning, and its chemistry, make people vulnerable to addiction. Our normal brain chemistry and functioning motivate us to repeat behaviors that are pleasurable (e.g., sex, eating). The repetition of these behaviors ensured our species survival. Unfortunately, it also makes human beings vulnerable to developing addictions to harmful substances and activities that are pleasurable.