Original paper (behind paywall I can't access): http://www.nber.org/papers/w16152
BBC summary (excerpted below): http://www.bbc.co.uk/news/world-south-asia-10746832
<begin excerpt>
The authors of the report by the Massachusetts-based National Bureau of
Economic Research say they analysed 15 months of data on military
clashes and incidents totalling more than 4,000 civilian deaths in a
number of Afghan regions in the period ending on 1 April.
They say that in areas where two civilians were killed or injured by
Nato's International Security Assistance Force (Isaf), there were on
average an extra six violent incidents between insurgents and US-led
troops in the following six weeks.
The report concludes that civilian deaths frequently motivate villagers
to join the ranks of insurgents.
"In Afghanistan, when Isaf units kill civilians, this increases the
number of willing combatants, leading to an increase in insurgent
attacks."
"Local exposure to violence from Isaf appears to be the primary driver
of this effect."
<end summary>
Michael