Hanh
If you are interested here is a study that shows the rise is also occurring across species. The factors are still being identified.
http://rspb.royalsocietypublishing.org/content/early/2010/11/19/rspb.2010.1890.full.pdf+html
The original study PDF ( from DenM's link ) is found here.
http://www.nature.com/nutd/journal/v2/n3/pdf/nutd20122a.pdf
