Many people think that once you have heard a PhD issue an opinion on something, you should yield to that person's assertion on the topic. But that is a dubious attitude. There are several reasons why it can be unwise to be greatly swayed by someone's opinion on something merely because he or she is a PhD, particularly if that person is talking about some general topic.
Reason #1: Today's PhD's are typically very specialized experts who have no special qualifications for speaking outside of their narrow subject matter.
Universities virtually never grant PhD's in large broad topics. They instead grant PhD's in specific fields such as astronomy, chemistry, physics, or something else. So a PhD who speaks outside of his exact field usually has no more authority on that topic that any layman knowledgeable on the topic. For example, when physicist Michio Kaku writes a book entitled The Future of the Mind, he is no more qualified to speak about it than a well-read layman, since it is a topic outside of his PhD, which is in physics. Similarly, when Stephen Hawking warns us about the dangers of artificial intelligence, we should pay no particular attention to his view, since computer science is not his area of expertise. The average computer programmer is more qualified to speak on such a topic than Hawking.
Even if a person is a PhD, it doesn't necessarily mean that he is qualified to speak on all topics relating to what he got a PhD in. Modern science is incredibly specialized. A 35-year-old biologist may have spent most of his career researching insects, and may not be particularly qualified to speak of the relation of the brain and the mind. An astronomer may have spent most of his career studying extrasolar planets, and may not be particularly qualified to talk about the universe’s origin.
Reason #2: You don't have to study a topic for many years to get a PhD – some people put in more time studying a topic to get only a bachelor's degree in it.
Many people think that you have to study a topic for 7 years or more to get a PhD. That is not at all true. Most graduate schools will admit candidates who have got a bachelor's degree in a topic different from the topic they are studying in graduate school. So a person can study Art History for 4 years, get a bachelor's degree, and then get admitted to a graduate school that will give him a PhD degree in Economics. Getting that degree may only require three years of study. The person will then have to do more work to get his PhD, but most of that work will probably be research work or thesis work, not taking courses. By the time the person is granted a PhD, he may have spent less time actually taking courses in his PhD topic than someone might have put in taking four years to get a bachelor's degree in some particular major.
Reason #3: In the Internet age, anyone can get specialized information almost as easily as a PhD can.
I remember about 35 years ago, I used to go to the MIT library to read cosmology journals that were only available in relatively few places (I wasn't a student, just a curious layman). Back in those days if you wanted to study the intricacies of some knowledge specialty, you might have to be a student or professor at a university, or perhaps someone willing to go read at a place like the MIT library. But now the situation is totally different. Anyone can read up the latest theoretical physics and cosmology papers for free at the http://arxiv.org/ server. Anyone can get tons of other specialized information online. With this democratization of information, we need not regard PhD's as being such special knowledge lords. The average American citizen today has more access to information on any particular subject than a well-connected PhD had a few decades ago.
Reason #4: You can become a PhD even if you have poor judgment.
Although PhD stands (because of obscure historical reasons) for “doctor of philosphy,” there is no test for judgment or wisdom to become a PhD. Having a title such as corporate vice-president is hard to get unless you have fairly good judgment, but a person with extremely poor judgment can get a PhD if he takes the courses and does the research.
Reason #5: Having a PhD doesn't guarantee that you now are thoroughly knowledgeable about the topic you got the PhD in.
I know someone who is a certified nursing assistant – someone with a CNA licenses. Periodically her employer must fill in a form that asserts she has been working as a nursing assistant. If that form isn't filled out periodically, she will lose her CNA license. But consider the case of a PhD. Even though becoming a PhD involves learning so much more complicated, there is no recertification requirement. Once a PhD, always a PhD. What happens is that many people get a PhD in some topic, and then hit the wall of the tough job market for PhD's. So a PhD may often become employed in some totally different industry. One example is that many physics PhD's end up getting jobs doing financial analytics on Wall Street.
What this means is that merely from the fact that someone has a PhD in a topic, you cannot tell whether they are currently thoroughly knowledgeable about that topic. For example, an author who claims to have a PhD in neurology may have been working the past 10 years in some entirely different field. Of course, if someone is both a PhD and a professor in some particular topic, you can assume he is currently very knowledgeable about it.
Reason #6: The narrowness of a PhD's studies may make him not particularly qualified to be speaking on general topics that require a great breadth of knowledge.
Let's consider a broad philosophical question such as: is there evidence of some paranormal influence or supernatural influence on the material world? Consider what you have to study to really be able to answer that question negatively in an authoritative way. You would need to have a good deal of knowledge of world religions, to be able to knowledgeably evaluate various claims of supernatural influence made by various world religions. You would need to study philosophy, to judge the validity of philosophical arguments for a divine creator. You would need to know a lot about physics, to adequately evaluate the claim that the fundamental constants of the universe and the laws of the universe are fine-tuned. You would need to also know a great deal about cosmology, to adequately evaluate claims that the universe’s sudden beginning is evidence for a divine creator. You would need to know quite a bit about chemistry, to properly evaluate the claim that the origin of life was too improbable to occur by chance. You would need to know about biology and evolution, to properly evaluate whether claims of intelligent design in earthly life can be plausibly overcome through Darwinian theory. You would need to know about neurology and psychology and philosophy of mind, to evaluate claims that the origin of human consciousness required some agent beyond that which evolution could have provided. You would also need to know quite a lot about parapsychology, to evaluate claims that certain observed anomalous phenomena (such as near-death experiences) are evidence of some paranormal realm or reality. In short, you would need to get a depth of knowledge about a wide variety of deep topics before you could authoritatively answer such a question.
Now if there were such a thing as a PhD in General Studies or a PhD in Philosophically Relevant Studies or something like that, that might be proof that someone can speak authoritatively on such a topic. But a PhD merely suggests that someone has mastered one of the many topics someone must master before speaking authoritatively on such a topic. The same type of situation holds for questions such as the future of man. To speak authoritatively on the topic, you might need to master history, sociology, computer science, genetics, international affairs, and several other deep subjects. But a PhD would at most show that you were well-versed in just one of these topics.
Reason #7: The opinions of PhD's may be heavily influenced by group norms or thought customs of clannish sociological groups they become parts of.
We often tend to think of academics as impartial judges, weighing issues of truth objectively like some judge considering a case. But we should remember that a PhD is very often under the strong influence of some little academic subculture that he becomes part of when he gets a PhD. For example, imagine you enter into graduate school at Dartmouth, studying evolutionary biology. It will become clear to you very soon that you are supposed to start acting like a Dartmouth evolutionary biologist, conforming to the expectations of your peers and superiors. This is a small geographically isolated subculture, a cozy little club with its own little list of taboos and norms that you will be expected to follow. If you don't follow such norms and avoid such taboos, you will be unlikely to find yourself with the academic position that you desire. So, quite probably, you will do what is best for your career, and start repeating the same “party line” being pushed by your superiors. Similarly, if you study for a PhD in economics at the University of Chicago, you will be pressured in numerous ways to start thinking and writing like a typical University of Chicago economist, rather than someone who advances economic opinions very different from those typically voiced at that institution.
Sociological factors such as these mean that a PhD is often someone who may be more like a corporation spokesman than a really independent thinker. What we often get from PhD's is a kind of “official party line” of some relatively small elite group that the PhD wants to be part of. If you spent 70,000 dollars to get a Yale PhD in say, neurology, would you start voicing opinions contrary to what your other Yale PhD neurologists are saying – or would you want very much to fit in with that little club, and meet the group norms of that cozy little clan? It is because of sociological reasons such as this that the opinions of PhD's are often not particularly enlightening. Such opinions are often just fancy rehashes of the local thought customs in the ivory towers of some academic institution. It might be better in many cases for you to seek out the opinions of well-informed independent-minded individuals who stand outside of some little academic culture where peer pressure, local thought taboos and group norms have such a strong effect.
Remember this: when you hear the opinion of Professor X of the Y Department at Z University, what you are hearing may be little more than a restatement of the thought customs (the ideological norms) of the Y Department at Z University. Such customs may be as disposable as the current fashion customs of the Kardashian clan.