Variety really is a motivating factor, according to exercise experts.
A big factor, according to Bjorn Hanson, an associate professor at New York University, is competition.
Dust blown up from the Sahara may also be a factor, according to Defra.
But the biggest factor, according to the teenagers themselves, is that they have come to understand the social moment.
The growing prosperity of citizens is also a factor, according to Brenden Jongman, a researcher at the VU University Amsterdam, who led the new work.
The global energy and utilities sector has been underperforming for the past five years because of a surprising factor, according to a recent report by EY (Ernst & Young).
Officials had not determined the cause of the crash, but today's steady and sometimes heavy rain was clearly a possible factor, according to law enforcement officers.
High ticket prices do not appear to be the primary factor, according to the report by the National Endowment for the Arts that is being released on Monday.
A more significant factor, according to hydrologist Paulo Carneiro of the Federal University of Rio de Janeiro, is the reliability of water supply.
Alcoa, the aluminum company, also pulled out of the coal coalition this summer, with climate policy being one factor, according to a spokesman.
Another factor, according to a classified C.I.A. analysis, is corruption with Iraq's security forces and Justice Ministry.
