Then there are all the unquantifiable things that motivate people all the time: ideas, beliefs, social status, altruism, reciprocity, social conditioning, values, worldviews, religion. Or even just acting 'irrationally' or against your own interests, which happens often enough. These factors are harder to study than economics, politics or outward behaviour.
It can be helpful to see rebels as social movements and not exclusively as economic groups, as this can help us better understand the range of motivations that compel people to pick up arms. Take stories of ISIS soldiers and brides who left rich countries to fight for the self-declared Caliphate, for example. The above theories (while providing insight into other factors) don't account for this.
Peace researchers are trying to find ways of quantifying these motivations so as to make better predictions, but it's difficult to find relevant
proxies.
A proxy, in this case, is an observable behaviour that might indicate an abstract motivation. We can't see hunger, but we can observe someone eating (or track food supplies that are available to a given community). The
gini coefficient is a famous example, which tracks income inequalities between individuals (known as
vertical inequality) in an attempt to find a proxy for general inequality in one society.
Henrik, in the
first lecture we had at PRIO, described a 'law' called
democratic peace theory, which is the notion that democracies are highly unlikely to attack each other. We don't know why this is (and why the concept is so stable), and researchers are trying to find the precise elements of a democratic society that makes certain kinds of conflict less likely.
This, however, does not apply to the likelihood of democracies fighting authoritarian regimes, it must be said.
Other hard-to-track factors are the social tensions unleashed when a state transitions into a newly democratic country (after the Cold War we saw lots of this). There are also transnational factors, border crossings, networks of who supports whom. Then there's all the backroom stuff that goes on that we don't have access too, and so can't factor into the equation.
That, and then there's the small pool of data to take from, and the relatively few conflicts that can be studied, and the limits of statistical analysis. You can't study conflict onset on a spectrum: either it happens or it doesn't. We can only make an educated guess as to what makes a war more or less likely. And even then you're watching for patterns (which can predict resurgent violence in West Asia) instead of outliers (no one thought that separatists would take East Ukraine).
To compensate for this, many researchers are trying to reach outside of the realm of quantitative research (numbers, statistics) in the realm of qualitative research (interviews, stories, subjective experiences of the subjects). There are many interesting collaborations with fields like anthropology, geography, the arts and so on.