[cross-posted at Liberty & Power]
I’m currently in the middle of the third volume of John Julius Norwich’s Byzantine trilogy (a generally fascinating work, I should specify, since I’m about to be critical), wherein he naturally enough attempts to explain the Byzantine Empire’s gradual decline and eventual fall. 
His explanations invariably focus on the personal qualities of various emperors and generals; indeed he seems to agree with the assessment of Emperor Ioannes VI, whom he quotes on page 293:
There is nothing more conducive to the destruction of a nation, whether it be republic or monarchy, than the lack of men of wisdom or intellect. When a republic has many citizens, or a monarchy many ministers, of high quality it quickly recovers from those losses that are brought about by misfortune. When such men are lacking, it falls into the very depths of disgrace. That is why I deplore the present state of the Empire which, having produced so many excellent men in the past, has now been reduced to such a level of sterility that today’s governors possess nothing to elevate them above those whom they govern.
Yet in passing, four pages later, Norwich mentions a rather different fact which one might think has some relevance to Byzantium’s decline:
Such wealth as existed in the impoverished Empire had … become concentrated in the hands of the aristocratic few, while the majority of the population could feel only indignation and resentment. In most Western societies, the cities and towns had gradually produced a flourishing bourgeoisie of merchants and craftsmen [but] in the Byzantine Empire this had never occurred ….
Surely Byzantium’s increasing economic desperation, inability to pay its mercenaries, and so on, has something to do with its failure to develop a prosperous middle class. What explains this difference between the Byzantine Empire and its Western neighbours? What features of Byzantine law, society, or culture were blocking economic progress? One might think that any historian attempting to explain Byzantium’s fall would show an interest in such questions.
But not Norwich. As he explains in the preface to the second volume:
If I tend to give economic considerations less than their due, this is because I am not an economist and a three-volume work is quite long enough already. Similarly, if I concentrate on the personalities of Emperors and Empresses rather than on sociological developments, I can only plead that I prefer people to trends.
Well, he’s perfectly entitled to focus on what interests him (though his contrast between personalities and trends strikes me as rather artificial). But the price of neglecting economic and sociological considerations, or treating them as some sort of optional add-on, is that one will inevitably fail to understand the events one is writing about.