It has been said that this is due to natural factors - for example, that lights in urban areas obscure sightings of lights in the night sky, or that rural areas include more people working outside who might misobserve natural objects. But the amount of the effect is enormous - a person in a sparsely populated area is over 100 times more likely to see a UFO compared to a person in an urban area with a population of over a million:
Furthermore, the effect is the same no matter what kind of UFO sighting is involved - nocturnal lights, daylight discs, or close encounters of whatever type:
And one could say that the effect is due to single-witness reports from lonely areas with no corroboration. But single and multi-witness reports show almost exactly the same trend:
So what kind of natural phenomena is only seen in less-populated, lonely areas?
What are the results of plotting the sighting rate on a map. We define the sighting rate as the number of sightings per population in each longitude/latitude grid square. The results for North America:
Interestingly, this difference according to poulation density accounts for the differences in reporting rate for different countries with equally-well developed press and government reporting systems (e.g. Australia, Canada, USA versus Western European countries). West European countries are more densely populated than the first group, and have correspondingly lower reporting rates, just as the above graphs would suggest.