Considering the ubiquity of addictive substances, the question may indeed arise as to why drug addiction, while relatively frequent, is not universal. The answer is perhaps the same as can be given to a similar question about overeating and subsequent obesity under the current conditions of virtually unlimited access to food in developed and even some third world countries. This access is a very recent environmental change. Obviously, mass access to abundant food is outside of what used to be the common food environment. The brain system controlling hunger and satiation that has evolved to motivate eating behavior determines the consumption limits, which, while varying in the population, generally determine the level of consumption that is compatible with or optimal for body function in what is termed the environment of evolutionary adaptedness, EEA (Bowlby, 1969). The evolutionary adaptation to food abundance, due to its recency, has not occurred to the degree that could prevent overconsumption and concomitant obesity and diabetes, even though they affect fertility and thus are under selective pressure. These conditions are well known to have reached epidemic
