Mike was always interested in being popular, so during his sophomore year in high school he began to use marijuana so he would fit in. After using it for little over a year, he began to need it to relax, and he also had to use more of it to get the same effect. He then started to use cocaine to get the same effect. He noticed that when he was not under the influence of the drug, he felt irritable and anxious. Judging only from what you know of Mike, what risk factor played the biggest role in the development of his addiction?