Following this post Bivariate probit model with sample selection, what would be an example of negative correlation between errors of selection and outcome equation?
I have also found an example where rho is negative: http://personal.rhul.ac.uk/uhte/006/ec5040/selectivity.pdf. Based on this example, unobserved factors or positive errors that make participation more likely (e.g. ability) tend to be associated with lower wages or negative errors of outcome equation. What could unobservables in outcome equation represent?