In an interview for Newsweek, Michio Kaku was asked about UFOs. Here’s his response:

Well, first of all, I think that there’s been a game changer. In the old days, the burden of proof was on the true believers to prove that what they saw last night was a flying saucer of some sort. Now the burden of proof has shifted. Now it’s the military, the military has to prove that these aren’t extraterrestrial objects.

He goes on to say that there are now multiple lines of evidence that need to be explained, and that this is the “gold standard” in science. It’s a good example of the fact that scientists and science communicators are not necessarily good scientific skeptics who know how to deal with fringe claims. There is an entirely different skill set and knowledge base necessary to deal with the UFO question than to explain physics to a lay audience. To illustrate, I am going to outline why I strongly disagree with Michio.

Let me start with two premises that I think should be noncontroversial. The first is that there is a phenomenon of alleged encounters with aliens – sightings, stories of abductions, videos and photographs, alleged government cover-ups, and others. We can stipulate that people see things they can’t explain and tell stories of alien encounters (sometimes under hypnosis). I will refer to this collectively as the “UFO phenomenon”. The second premise might get some pushback from believers, but I think is completely reasonable – there is no smoking gun unequivocal evidence that aliens are visiting or have visited the Earth. If there were, there would be no debate. You can still believe that the evidence favors the conclusion that we are being visited, that at least some of the UFO phenomena is produced by actual aliens, while accepting the premise that there is no undeniable proof.

If you accept these two premises – there is a UFO phenomenon in the absence if iron-clad proof – then we can talk about competing hypotheses to explain these premises. There are two main contenders. The first is what I have called the “psychocultural” hypothesis. This contends that all UFO phenomena are a mix of misidentified natural or terrestrial phenomena, the many mechanisms of self-deception, cultural belief, delusion, and opportunism. The second is the alien hypothesis, that at least some UFO phenomena are alien craft, encounters with aliens, crashed saucers, or alien experiments. There are other hypotheses (the tongue-in-cheek psychic bigfeet from the future), but they are fringe enough that we can ignore them for now, and in fact I feel they can be subsumed in this debate under the alien hypothesis.

Scientists, and certainly skeptics, have long maintained that the burden of proof lies heavily with the alien hypothesis. Does Michio have a point that this burden of proof has now shifted? I think the answer is a clear no. This burden of proof is based largely on Occam’s razor – when there are competing hypotheses, the one that introduces the fewest new assumptions should be preferred. I would state it more precisely as – the hypothesis that has the lightest assumption burden. This combines both the number of required assumptions and how big or unlikely those assumptions are. The alien hypothesis has the burden of a massive assumption, that aliens are visiting the Earth. The psychocultural hypothesis, on the other hand, has a very light assumption burden. It is built out of mostly entirely known and fairly well understood psychological phenomena. We really don’t have to invent anything new, just extend our understanding of the frailty of human perception, memory, and belief.

We can compare these two hypotheses in other ways. One is to think about what world we are living in under either hypothesis. First, is it possible or plausible that the UFO phenomenon can exist as it is in the complete absence of alien phenomena? For me that’s an easy one – yes. We have lots of examples of strong cultural beliefs in the absence of real underlying phenomena. That’s pretty much the human condition. On the other hand, is it possible or plausible that we are living in a world where we are being visited by aliens but still have a complete lack of definitive evidence? I would say not impossible, but increasingly implausible.

This gets to another factor that I think strongly favors the psychocultural hypothesis – time. It may have made sense in the 1950s, at the dawn of the modern UFO phenomena, to hold out belief that aliens were teasing us with their existence and would soon reveal themselves. Or that we would eventually come upon some unambiguous evidence. Or that the US government already had that evidence but was keeping it under wraps. If you talked to believers in the alien hypothesis at that time (and you can read articles and interviews) there was a sense that definitive proof, one way or the other, was just around the corner. Same for believers in the 1960s, and 1970s, 1980s, 1990s, 2000s, and now. This is partly why UFO enthusiasm waxes and wanes over the decades. Each generation becomes fascinated with the possibility of aliens, but then most enthusiasts fade away as the big reveal never comes, leaving only die-hards who will keep the flame going until a new generation can start the cycle over again.

Each special pleading argument for why we have only tantalizing evidence without proof becomes increasingly untenable over time. The “aliens are teasing us to prepare for the big reveal” idea really strains credulity after 70 years. The government conspiracy excuse also becomes exponentially less plausible over the decades, through multiple administrations, and entirely new generations of people who must be keeping the secret. Even the motivation for such secrecy starts to become absurd over time. When you consider that the UFO phenomenon is international, a massive conspiracy becomes even less credible.

Time also works against the alien hypothesis in other ways. As many people have pointed out, recording technology has dramatically improved since the 1950s, and smartphone cameras and videos are now ubiquitous. However, according to the psychocultural hypothesis, ambiguity is a feature (not a bug) of UFO evidence (just like Bigfoot, which is how it has earned the nickname “blobsquatch”). As I am fond of saying – the ambiguity is the phenomenon. Therefore, the psychocultural hypothesis predicts that as technology improves and becomes more common, the evidence will remain ambiguous. However, if the alien hypothesis were correct, then we would reasonably predict that as technology improved and everyone is walking around with cameras, the photographic and video evidence for aliens would likewise improve. But it hasn’t. We are still seeing lights and blurs, just more of them.

Finally, we still need to take a close look at each piece of alleged evidence. At the end of the day, evidence is king. This gets back to the second premise. When we do look at the best evidence put forward for the alien hypothesis, the evidence is weak, ambiguous, and highly problematic. Whenever we do a deep analysis of any bit of evidence, it evaporates. At least there is no way to eliminate non-alien phenomena. We have more and more elaborate explanations for the blurry evidence, but at the end of the day we still have only blurry evidence, implausible stories without evidence, the occasional hoax, and lots of excuses. The psychocultural hypothesis has faired extremely well over the last 70 years, while the alien hypothesis has been spinning its wheels.

So no, the narrative has not shifted. I have personally been following the UFO phenomenon for over 40 years, and started out predisposed to the alien hypothesis. I experienced first hand the creeping disappointment when all the amazing predictions of the UFO promoters failed to come true. Of course over this time I was deepening my scientific knowledge and critical thinking skills, becoming a scientific skeptic. That has provided a much more enduring explanation for the UFO phenomenon, one that has stood the test of time.