Interaction term between a dummy variable and a quadratic term

Hi,

I am using stata 12 and running a multilevel model. I want to test three hypotheses: 1) Individuals' income has a curvilinear (inverted U-shaped) relationship with protest participation1) Individuals with economic motives (dummy variable) have a higher probability to participate in protest and 2) the effect of economic motives is only significant for middle class individuals (in terms of income). Thus, the central variables in my model are economic motives, income, and income^2.

First, I run the model without the interaction effect between income and motives, then I add the interaction effect. However, since income is hypothesized to show a curvilinear relationship with protest participation, I include the squared term of income but I am not sure how to include the interaction term. Do I have to interact income*motives? or income^2*economic motives?

This is the code I use for the regression without interaction terms:

xtmelogit economic_protest income_PPP_adjusted2011 i.motives income_PPP_adjusted2011_sq i.insurance ///
i.maritalst i.female age i.urban || ctrycode:, variance

Many thanks for your help,

Barbara