Evolution of "Per Capita Income"

Traditionally men (women rarely had any value) were viewed more as a silver earned by selling them as slaves or how large an army could be drafted from the region. The value of human life was so low in the medieval ages that people stopped buying slaves (you could inherit a fief and its serfs for free so why pay). Go East for riches campaigns during Crusades (and later go west during settlement of America) was probably the only historic references where establishments tried to place a value to a free-man’s life.
Simply put if human life had any value, you would have emigration checks and not immigration checks. Till recently many middle east regimes did not encourage university education because graduate students would destabilize the regime. (much like early industrial age governments saw factory workers as a threat). So development of human capital was always a private endeavor and only recently has it been institutionalized.
Similarly land traditionally used to be viewed in terms of how much taxes it could yield (cities, ports) or how much grain/men/resource it could produce. A land surveryer Sir William Petty in early 17th century tried to resolve this by coming up with GDP, a unifying parameter to place the value of the land & tax it aptly.
Now the question is how did the concept of per-capita income evolved and why? Except for the first half of 1940s (towards the end of of 2nd world war) there has never been a global shortage of manpower, but the earliest references date back to 1934… at that time the world still was recovering from great depression and had double digit urban unemployment (rural unemployment was never measured and is hard to compute even today)
Also what is the relevance of per-capita income? Given a choice between 2 regions both producing the same GDP, would a statesman/politician prefer a larger population base or a smaller? (smaller or larger per capita?)

The cost of governance

History has taught us that the more the lands, resources and trade routes a regime controls the richer the state is. Wars traditionally were a very profitable empire bringing loot and access to resources which could fill the state’s coffers for ages. All this was true till about the early 20th century (First World War to be precise). I wonder what has happened since because today:
1. The richer and more advanced the country is, the more debt laden it is. Poor and developing countries are bailing out the rich, while the rich are unable to muster enough currency to every pay the interest on these debts.
2. It does not matter who wins the war, the participating nations are never able to recover its cost. I might be wrong but the reason why there are so few wars in recent history is because no country thinks they can afford one.
The question is till how long will China & Africa continue to fund the prolerific
Update: Right from the ancient time to Great War (world war 1, 1914), the army was under Ministry of War. People never talked about percentage of GDP that was being spend on army, navy or airforce. Instead each army was like a business enterprise. The loot & plunder or protection taxes always exceeded the expense to train & equip an army. Also after a elongated long conquest, the army general usually paid tribute to the emperor (often as Spanish Steps, Colosseum and glorious building). As mentioned in this post, only in the recent time has the defense being shown as a cost center.