“…In a first model (model 1), we express varietal diversity as explained above, namely in terms of the total number of cotton varieties grown on a farm. Yet there are also more specific indices that can be used to express varietal diversity, such as the Margalef index of varietal richness or Simpson's evenness index that both account for the size of the farm (Di Falco and Perrings, 2003;Benin et al, 2004;Nagarajan, Smale, and Glewwe, 2007;Di Falco and Chavas, 2009 (Karihaloo and Kumar, 2009;James, 2012). We expect that the number of Bt varieties locally available in the seed market will affect the diversity on Bt adopting farms, so we use an interaction term between the number of approved Bt varieties and Bt adoption.…”