I tend to divide fringe beliefs unsupported by science into two groups.
The first kind includes harmless, titillation-centered pseudoscience dealing with the pursuit of nonexistent phantoms, like the UFO phenomenon, ghosthunting, the search for mysterious creatures like Bigfoot, and (my personal favorite) the Hollow Earth.
I must confess, I'm something of a fan of these theories because I deeply sympathize with the desire for them to be true. No one would be more excited than me if scientists found a previously undiscovered North American primate! Even today, I have daydreams about encounters with alien life, mermaids, and strange monsters, as well as becoming a witch or having mind powers. At the same time, I understand through critical thinking there is no there there behind theories of this type, and their supporters tend to have an extremely naive definition of what constitutes proof.
The second type of pseudoscientific theory is much nastier, because they are discredited beliefs with an extremely negative and manipulative influence on discourse, that usurp science and attempt to manipulate its fundamental doubt and skepticism to undercut it. Global warming denial is one such theory, as is Creationism and Intelligent Design. One of the oldest and slowest to die is race theory, or the idea that the human race is broken up into distinct racial subgroups, some of whom are culture creators and others are culture destroyers.
The reason we know Race Theory to be unscientific is because differences in phenotype (external appearance) do not indicate differences in genotype (genetic traits). Modern Europeans, for instance, according to modern genetics, share two-thirds of their ancestral DNA with African groups, despite the fact the external appearance of these groups could not be more different. Africans and Europeans tend to have similar blood types. When looked at from a view of human history involving migration, this is not only not shocking but entirely expected, as Africa and Europe are close to one another.
Another strike against racial theory is how non-immutable and truly undefinable racial categories are. Some classification systems include 3 main human races, others go up to 80. Species are a very real category, but subgroups of this type are not. In fact, no genetic test can reveal with certainty what race a person is. The reason is that there is greater variability within groups than between groups.
In fact, one of the greatest signs of how nonscientific racial categories are is the arbitrary determination of what groups are "pure" and which are "mixed." The earliest racial schema is from around 1500, from around the start of the Age of Exploration and the discovery of the New World. Racial schemes after this time list a few "new" groups that began to exist in the New World: mulattos, or mixed African/Europeans, and mestizos, or mixed European/Indians. The question is, in the long term scope of human history and migrations and intermingling, groups like the mulattos and mestizos are considered blended groups but others are not? The modern English population for instance has traces of Angle, Saxon, Danish, Norman, Roman and Celtic, yet the English are not considered a "blended" group.
What is really interesting is how group characteristics have vanished into greater populations, and many populations no longer exist. For instance, many remains of modern humans have characteristics not similar to populations in the area. Kennewick Man, a skeleton found in Washington and dated to around 9,300 BC, has been famously compared to sexy deep voiced Star Trek actor Patrick Stewart. There was another Homo Sapiens skeleton found in Herto, Ethiopia and dated to 155,000 (!) BC, which more closely resembles Australian Aborigines than any modern group.
In fact, I have a pretty funny personal story about the degree of genetic variability within groups. As someone close to the biology department, they were doing testing of people and genetic markers in populations, and they paid students $30 and a Starbucks gift card to do some blood tests.
I jumped at the chance to help! It all sounded very exciting. For those that don't know, I am of Cuban descent and my family tree is almost equally divided into thirds between three racial groups: Southern European (specifically, Galician), African, and Chinese.
(That last one tends to surprise people that don't know much about Cuban history, but Havana has one of the largest Chinatowns except for Chicago and San Francisco in this hemisphere.)
I expected that my test results would show I have genetics in common with Mediterranean European, African or Chinese groups. Wrong! Apparently, most of my genetic markers are held in common with (of all the damn populations in the world) the Basque. To my knowledge, I don't have a single drop of Basque blood!
One of the vilest non-science attitudes is the view that there is something apelike about African blacks. This is extraordinary because, in the view of most physical anthropologists, the various groups we call "African negroes" were actually the most recent of all human populations, dating to 9,000 to 7,000 BC. For most of its history, the African continent was inhabited by the ancestors of the Hottentots, Bushmen and Ituri ("pygmies"), who are no more related to African negroes than Polynesian Islanders are.
To be clear: I am not going on about the illegitimacy of race because I want to get "street cred" for being "hip" and "progressive" and "liberal." I am pointing racial theory is pseudoscience because there is no evidence whatsoever to support it.
However, it is important to see how theories of race change with social forces in society as a whole. There have been times in American history for instance where the Irish and Italians have not been considered "white" at all. When both groups acquired political power and joined the middle class (and internalized middle class values and culture), they ended up becoming just another kind of suburban white people. This is my general point: race is a social and (at times) economic category, not a scientific one. Racial categorization is as much "junk science" as creationism.
Finally, I think it is important to point out that the discipline of physical anthropology is both fascinating and a legitimate field, and the study of different groups and populations is an intriguing one. One of the most interesting results is the discovery that sickle-cell anemia is incredibly common among West Africans. The reason? The gene that gives resistance to Malaria is actually closely connected to the mutation that results in sickle-cell.