ONE of the puzzles of the modern world is why we humans are growing so tubby. Maybe these two mice offer a clue.

They’re genetically the same, raised in the same lab and given the same food and chance to exercise. Yet the bottom one is svelte, while the other looks like, well, an American.

The only difference is that the top one was exposed at birth to just one part per billion of an endocrine-disrupting chemical. The brief exposure programmed the mouse to put on fat, and although there were no significant differences in caloric intake or expenditure, it continued to put on flab long after the chemical was gone.

That experiment is one of a growing number of peer-reviewed scientific studies suggesting that one factor in the industrialized world’s obesity epidemic (along with Twinkies, soda and television) may be endocrine-disrupting chemicals. These chemicals are largely unregulated — they are in food, couches, machine receipts and shampoos — and a raft of new studies suggest that they can lead to the formation of more and larger fat cells.