[amelius]

Can someone confirm that theory and provide evidence?

[hwbehrens]

This correlation is often referred to as the 'hygiene hypothesis' by researchers. The underlying cause is still being understood; you can find an article comparing two theories here [0].

[0]: https://www.nature.com/articles/nri35095579

[rajup]

There are plenty of studies linked here: https://en.wikipedia.org/wiki/Hygiene_hypothesis