Free will is an important aspect of many denominations of Christianity. As C.S. Lewis explains "God created things which had free will. That means creatures which can go either wrong or right. Some people think they can imagine a creature which was free but had no possibility of going wrong; I cannot. If a thing is free to be good it is also free to be bad. And free will is what has made evil possible. Why, then, did God give them free will? Because free will, though it makes evil possible, is also the only thing that makes possible any love or goodness or joy worth having." However, the more we learn about the forces influencing our decisions, the less room there is for such freedom of will.
Genes hold the information to build and maintain our bodies' cells and pass genetic traits to offspring. Since our brain is comprised of cells, it should not come as much of a surprise that genes can influence our personalities. In fact, twin studies have shown that intelligence, stress resistance, risk aversion, achievement-orientation, the need for interpersonal intimacy, and religiosity have a strong genetic basis. Other studies have also indicated there are specific genes which are associated with chemical dependence. Thus, genes likely play a large role in the decisions we make.
The word epigenetics literally means "on top of genetics," and it refers to the phenomena wherein environmental factors that affect our biology change how our genes are expressed. These factors may include the foods we eat, the toxins to which we are exposed or even emotional stressors. For example, women who were pregnant during the 9/11 attacks and experienced post traumatic stress disorder ended up having children who were prone to anxiety.
Beyond genetics and epigenetics, our experiences play a major role in the development of our personalities. For example, children who were raised without a father (or a 2nd parent) are 20 times more likely to have behavior disorders, 14 times more likely to be rapists, and 20 times more likely to end up in prison.
A Complex System
Our personalities, and thus the decisions we make, are ultimately derived from a number of genetic, epigenetic, and experiential influences. Given humans have over 20,000 genes, and since we can experience a near infinite quantity of environmental influencers, it is hard to know the degree to which any gene or environmental factor may affect a particular decision. Even when looking at large groups of individuals, scientists have a difficult time sorting out which personality traits are driven by nature vs. nurture. Often, their findings aren't very clear-cut. For example, when it comes to smoking tobacco, women tend to be influenced more by environmental factors such as peer pressure, whereas genes play a greater role for men. In addition, studies show that variation in IQ has little to do with genetic factors among those living in extreme poverty, yet, they are a major influence among those who are affluent.
The Rational vs. Intuitive Mind
Some may argue that while various forces can influence their decisions, it is ultimately their free will that allows them to make choices. After all, we feel like we understand the reasons behinds our beliefs and decisions, since we often make choices after deliberation. However, modern psychology has determined that our conscious mind has less control over our decisions than we assume.
In the dual process model of cognition, decisions and beliefs derive from a combination of rational (conscious) and intuitive (subconscious) mental processes. The intuitive mind is quick, automatic, emotional, and based primarily on mental shortcuts. Conversely, the rational mind is slow, effortful, rule based, and logical. Most importantly, the rational mind can counteract the decisions of the intuitive mind, but doing so usually requires effort. Thus, by default, the rational mind tends to defer to the intuitive for most decisions. In addition, the rational mind tends not to form judgments independent of the intuitive mind. Instead, using psychologist Jonathan Haidt's comparison, the intuitive mind is like an elephant and the rational mind is like its rider. For example, in a study conducted by psychologists Daniel Kahneman and Amos Tversky, judges with over 15 years of experience on the bench were presented with a case regarding a woman who had been caught shoplifting. Before being presented with her case, however, they were to roll a dice that was loaded so that it either came up with the number 3 or 9. Those judges who were presented with a 3 recommended the woman be sentenced to 5 months in jail on average, whereas the ones that saw a 9 suggested a sentence of 8 months on average. Thus, their intuitive minds anchored the sentences to the numbers presented by the dice, but their rational minds corrected the sentences directionally from the anchor. Like an elephant, the intuitive mind goes where it wants, while the rational mind tries its best to steer.
Given that our decisions are influenced by genetic, epigenetic, and experiential factors that are out of control, and since our minds are guided primarily by subconscious processes beyond our awareness, it's hard to argue that we have true freedom of will. Maybe one could argue free will if we were somehow capable of understanding all of the influences that guide our subconscious mind, but this is far from the case. Thus, while we can make conscious choices, and it may appear that our decisions are freely made, free will is merely an illusion.
Fantastic lecture by Sam Harris regarding free will
Great article on the complex nature of nature vs. nurture
Good epigenetics article
Great article regarding rational vs. intuitive processes