Further, studies have not tried to determine at what stage in the conflictual process external parties are likely to intervene in the politics of another nation. An association between high levels of conflict and external military intervention has been uncovered, but the evolution of internal conflict over time and the point at which intervention has occurred have not been examined. There has also been reluctance to investigate other variables that would help to explain the particular timing of an external military intervention. For instance, to what extent does social polarization between internal groups contribute to a greater probability of external intervention? How do the appeals of internal groups for outside aid involve external parties in resolving the political disputes of another nation? The answers to these issues require an analysis that considers the dynamic nature of political behavior within and across nation-states. Cross-sectional designs, which characterize much of the research on foreign-domestic conflict linkages, are not capable of capturing this phenomena.