You know that friend who always loves movies he picked? Or the one who's thrilled about her brunch spot but never yours? Don't be too hard on these people, new brain research suggests. The quirk is in their genes.

The phenomenon is fairly well known. It's called "choice bias," and everybody has it to greater and lesser degrees. Here's another common example. You shop for a new car, and you decide to buy a Toyota. Ultimately you would have been equally happy driving a Nissan. But for reasons I'll soon explain, your choice amplifies the reward, and suddenly you are a huge Toyota fan. Interestingly, you would have been less enthusiastic about the exact same Toyota, science shows, if you had won it in a lottery.

On Thursday, neurologists at Brown University announced new revelations about the causes of choice bias (and perhaps about the nature of personality itself). Differences in a single codon in the alphabet of a particular gene (DARPP-32) have been shown to make people more responsive or less responsive to the intoxicating thrill of dopamine reinforcement. Here's what that means:

When people make good choices, the brain automatically rewards itself with a flood of dopamine, the pleasure hormone. It does this to increase the chances of making good choices in the future. But there's a side effect of this, as the Brown news release notes: "Along with that process of reinforcing the action of choosing, the value placed on the resulting reward becomes elevated compared to rewards not experienced this way." Your brain puts outsize value on the fruits of your decisions.

Much of this was already known. What's new is the role of genes in determining a person's susceptibility to choice bias. The scientists devised an experiment to test how strongly 80 study participants experienced choice bias. Each was told to pick random Japanese characters that were affiliated with greater or lesser rewards. Sometimes characters were simply assigned, with no choice involved. The researchers also took DNA samples from each person's saliva.

Some participants reacted with more choice bias than others to the more strongly rewarded characters. And, as the scientists predicted using computer models, those were the same people who had the genetic predisposition to dopamine's sweet succor. Depending on your outlook, this may be troubling. It means scientists predicted the behavior — and more broadly, an endearing and/or annoying personality trait — of 80 people using nothing but a computer and some DNA.

In theory, now geneticists can tell new parents whether their infants will have strong choice bias. No doubt more of this kind of research will link more genes to their corresponding personality traits. Studies involving twins have famously declared that personality is hard-wired more than it is acquired. All of which may suggest the mysteries of our brains, our most human stuff, is predetermined, encoded and, eventually, available in our spit samples for others to read.

Source: J. Cockburn, et al. Neuron. 2014.