I was thinking about mulberries just now. I distinctly remember the first mulberry I ever ate. I was about thirteen, it was the beginning of summer and a group of middle school boys were riding bikes on paths through some woods near the neighborhood. It was sweaty work and we stopped to catch our breath under a mulberry tree. The branches were heavy with berries, and some were low enough that we could try them. The very dark ones were sweet, the lighter ones bitter. We cleaned the lower branches of dark berries, climbed the tree and cooperated to get every berry that could be reached.

Decades later I moved into a house and there was a mulberry tree on the property. I did not recognize the tree or it’s leaves. Only in the early summer when the green fruit began to appear did I recognize that it was a berry tree. It had been decades since I had seen a mulberry, but I knew that there would be a sweet treat for us every morning as the berries blackened.

Such is the power of sugar. There are millions of things that have happened to me between that climb in the berry tree of my youth and now that I could not recall with the clarity of what those berries looked and tasted like, from a single day forty years ago! One look at a green pre-mulberry and I instantly was transported back across decades to memory of taste and satisfaction. This is a sure sign of the ability of the brain to ensure the survival of the species–any good food source is instantly locked into permanent memory.

Addiction begins with such knowledge. Addiction to sugar happens to be a feature of the human mind, one that will ensure that a source of sugar will not soon be forgotten. This feature is a relic of a time when sugar was not available in every bite of food we would eat. Back in our prehistory we would only get sugar from fruit and only fruit for a single season out of four. If you forgot and had to relearn what was sweet every year, that would be a serious disadvantage compared to the other beasts that do remember. Fortunately we are constructed such that an important thing like where to find sugar is never forgotten.

Obtaining that first mulberry was by chance. Every subsequent trip to the tree over the years was prompted by desire. The effect on the mind of the sugar in the berry was taken advantage of by the tree, so that I and all other sugar desiring animals would spread its seeds through our desire. Every time I eat sugar the want of it means more to me than the actual act of eating. Once the food is in my mouth I scarcely think about taste after the first taste. A tray of pastry on a table calls out to me every moment that I do not eat from it, but once I do–in an instant the desire is gone when the food is. The pastry never tastes as good as I imagined it would. The desire is the most important thing, the thing that has to be satisfied.

So desire is really the big wheel in all our goal-directed activities. And addiction is no exception. The critical role of desire in the brain has been the focus of research in Berridge’s lab for well over a decade. Berridge was the first to argue that addiction is about wanting, not liking— desire, not pleasure— while the rest of the field has been catching up slowly. The low profile of pleasure in addiction explains why Natalie kept shooting heroin, Brian kept smoking meth, and Johnny kept drinking, long after the enjoyment dimmed to an ember of its former glory. And why smokers are rarely heard to celebrate the pleasure they get from smoking— at least after the first cigarette of the day. Even the satisfaction afforded by relief doesn’t remain in attention for long. But the drive to get that relief, to acquire it, especially when it’s been out of reach for a while, takes on colossal proportions. Not that pleasure isn’t important. There’s a reason why all species of fruit have evolved to produce sugar: so that mammals will eat them and spread their seeds. Pleasure is great for triggering desire— I want more! But once that connection is made in Act 1, Scene 1, the audience turns its attention almost exclusively to desire.

Lewis, Marc (2015-07-14). The Biology of Desire: Why Addiction Is Not a Disease (Kindle Locations 2704-2713). PublicAffairs. Kindle Edition.