No-one knows why: theoriesrange from the notion that it might have an antibiotic effect to the idea that it makes algae less appealing to munching zooplankton.
Theories about why this is happening range from higher levels of obesity, higher density of calories in processed foods, and physical inactivity, to chemicals in food and water.
Partisan paranoia: Finally, a new poll found that absurd conspiracy theories are sadly alive and well in the USA. These range from the self-evidently absurd (an alien race of lizard people secretly run the world) to old chestnuts like the moon landing was faked.