Evolution is "only a theory." If we evolved from monkeys then why are there still monkeys? The climate may or may not be changing but if it is it's not the fault of humans. How can there be global warming when we're having such a cold winter? I am convinced that vaccines can cause autism. Gluten is unhealthy for a lot of people to eat.
I have heard the above statements, and many more like them, over and over for many years. The drumbeat doesn't seem to be waning at all as the body of evidence continues to grow that should debunk every claim of the science deniers. But why should that be so? Why do people who are otherwise mature, intelligent adults continue to deny the evidence in front of them? Why do so many people deny science?
First, let's start with a definition of science to see what it is they are denying. Science is actually two things: 1) Science is a methodology used to discover, through the use of empirical data, reasoning, experimentation, and logic, how various facets of nature work, and what predictions can be made by applying the discoveries to new situations. 2) Science is also the body of knowledge obtained from such methods and discoveries.
Most science deniers don't have a big problem with the first definition. But if those scientific methods lead to discoveries that conflict with their worldview or religious beliefs, then we have a problem. People, even smart people, have gathered throughout their lives personal data and experiences that have led them to formulate a set of beliefs. Many of these beliefs have been drilled into their heads by their parents, pastors, relatives, and peers. Those beliefs form their worldview - a mindset that governs their perspective on such things as politics, religion, and even their personal philosophy of life. Most of the time when new facts are presented they are accepted without problem because the new information does not conflict in a major way with a person's worldview. If you had always understood that matter exists in three states - solid, liquid, and gas - because that's what you learned in school and it conforms with your observations, you might be skeptical if someone tells you that there are actually four states of matter. But if that person then defines plasma to you and gives you examples of where you might find plasma, you won't have much trouble assimilating that new information. It hasn't altered your worldview tremendously.
On the other hand, if you grew up believing that God created the world in six literal days and that every living species on the planet was created by God personally, then any new knowledge that tells you that that belief isn't true becomes problematic for you to assimilate. That's partly because it represents a major change in your understanding of how life got here. But more importantly, it injects uncertainty into your view of what happens to your immortal soul when your life on Earth ends. For some people, it takes a leap of faith too big to make; never mind that science is not based on faith.
The actual term for a person's feeling of intense uneasiness when confronted with new information that contradicts his established worldview is cognitive dissonance. This generally elicits behaviors that tend to undermine and reject the conflicting evidence, leading to cognitive dissonance reduction. The mental uneasiness is sometimes so great that facts and evidence no longer really matter. Something must be wrong with the way the evidence was obtained or interpreted, because it just can't be true. Sometimes the cognitive dissonance is so great that it actually leads to a hardening or firming up of the already-held worldview. This is one reason why after a tragedy the faith people have in their personal god is increased.
Another very strong influencer of people's wrongly-held beliefs is anecdotal evidence. Since anecdotal evidence seems rational and since it is usually quite easy to understand, it leads to widely-held beliefs that are scientifically unfounded. Remember, in the definition of science, I said that science makes discoveries using empirical evidence, not anecdotal evidence. Empirical evidence is the type of evidence that can be measured and repeated. The earth's average temperature is rising. That can be measured independently by climatology labs all over the world. Caron dioxide levels in the atmosphere are increasing. The evidence for this comes from the direct measurement of CO2 by labs in many countries. Carbon dioxide gas traps heat given off by the earth. The evidence from this comes from experiments that directly measure the temperature of the air above a heat source in a controlled environment where CO2 has been elevated compared to an environment where it has not. The logical conclusion would be that the earth's temperature, on average, will rise and fall as a function of the amount of CO2 present. Of course there are other factors but you get the idea of how a hypothesis in science is created.
Anecdotal evidence doesn't work this way. There are no measurements and no experiments. Anecdotal evidence comes only from the stories, or anecdotes, that people tell based on their own perceptions or the perceptions others have related to them. For example, "I've personally noticed that the winters here have been getting colder over the past few years. My grandfather told me that he remembers often going to school in December wearing only a t-shirt and jeans. My kids always require a coat, or at least a jacket in the winter. Therefore, global warming must be a hoax."
That sort of evidence does not pass scientific muster and cannot be used to make accurate predictions about what is likely to happen in the future. Being able to throw a snowball in the winter does not mean that global warming isn't real. The fact that you have heard of more than one child developing autism after having a vaccine doesn't mean that vaccines cause autism. The fact that you and your friend got sick after eating a gluten-laced piece of cake doesn't mean that gluten is a bad part of your diet. Just because you know of people who get headaches every time they drink a diet pop doesn't mean that sugar substitutes cause headaches.
Empirical evidence that has been gathered in controlled experiments always indicate that the anecdotal evidence against climate change, gluten, and vaccines is completely erroneous. More than a century of empirical evidence gathered about evolution supports the conclusion that life on earth got here through the process of evolution by natural selection and completely debunks the notion it arrived all at once through the action of special creation.
If you believe in special creation or the detrimental effects of gluten or vaccines, then when you are confronted with loads of empirical evidence that debunk those beliefs, your cognitive dissonance reduction behaviors kick in and you start denying the hell out of the real science. And, like kicking the smoking habit or losing weight, it's very hard to stop the denying. But just like those other bad habits, stopping the denial will turn you into a better, more informed, and less angry person. Give it a try.