Smoking Is Dumb: Young Men Who Smoke Have Lower IQs, Study Finds:

This research, conducted by Prof. Mark Weiser of Tel Aviv University's Department of Psychiatry and the Sheba Medical Center at Tel Hashomer Hospital, focused on 18-21 year old men enlisted in the Israeli Army, in the largest ever study of its kind.

The aim was to find out a relation between cigarette smoking and IQ levels. Choosing to study soldiers had two benefits; one being all the men were considered fit and healthy due to pre-screen tests for Army permission. And the second benefit is knocking away the common thought that most smokers are from poor, uneducated families, as the subjects came from diverse economic backgrounds. 

Out of the approximately 20,000 men, 28 % smoked one or to cigarettes a day, 3 % considered themselves ex-smokers and 69 % said they had never smoked.

The average IQ of a non-smoker in the classified age group is 101; whereas the average IQ of those who smoked was approximately 7 grades lower at 94.

Another interesting find was related to twin brothers. In the subjects which one twin smoked and the other didn't, the whole philosophy of this research is proven; as in twins, the genes are almost identical. The results showed that the IQ levels of the non-smoking twin is slightly higher.