This is a great question and one that is becoming increasingly important as neuroscientists and psychologists increasingly suggest that their research is showing that free will is an illusion, a claim I call 'willusionism', and as increasing evidence comes in that shows that these willusionist claims, which the media loves to report and exaggerate, can have (at least short-term) negative effects on people's behavior. I have lots to say on this topic, some of which I say in papers on my website if you want to hear more, but here's a brief take on the issues.
Suppose you believe that free will requires the abilities to make choices based on conscious deliberation and reasoning and to control your actions in light of these choices (against internal desires to do otherwise and without external contraints preventing your action). Free will requires that your conscious self can make a difference in what happens. You may also happen to believe that the only way to have such free will is to have a non-physical mind that is not subject to the laws of nature.
Now, suppose scientists come along and tell you that they have shown that we do not have free will. What will you take that to mean? I think it is likely that you will take it to mean that your conscious self doesn't make a difference in what happens, that you are just a spectator observing the outcome of mechanistic processes beyond your control. And that's the way some willusionist claims are presented. I think it is this response that most likely leads to the 'bad results' of willusionist claims--i.e., the findings that people cheat more, help less, are more aggressive, etc. (studies by Vohs and Schooler and Baumeister and colleagues).
The problem is that the scientific studies have not shown that conscious deliberation and reasoning and self-control are illusory. What they have shown is that there is increasing evidence that we do not need to posit a non-physical mind or soul to explain these cognitive abilities and that, in certain experiments with a specific setup (e.g., Libet's), you can find brain activity that predicts actions before people are consciously aware of their intention to act. But these findings do not show that the brain processes involved in conscious deliberation and decision-making never play a crucial causal role in what we do. It'd be very surprising if the brain processes involved in your conscious planning about what you will do with the rest of your day had no effects on what you end up doing later today.
It turns out that my research on people's beliefs about free will suggests that they are not wedded to a dualist conception of free will (or even one that conflicts with determinism). So, there are (at least) three problems with the way willusionist claims are presented that may account for the bad results. First, the scientists may assume people believe free will requires X when people don't believe that, and they think that their research shows that X is false. And second, when you tell people they lack free will, they will interpret it to mean they lack what they think free will is--i.e., conscious and rational control (Y). And when people believe they lack Y, that likely makes them less likely to engage their capacities to consciously and rationally control their behavior. But third, the scientific evidence has not shown that we lack Y.
There are a lot of issues going on here, and I haven't really explained why we have reason to think humans have some degree of free will even if determinism or naturalism is true (which may be what you wanted to hear). But I hope this helps.