Good question!
You and I both agree on what constitutes an inferior culture ... pretty much everyone in the world does, including those living in one. Plenty of available metrics ... literacy, in particular literacy for women, is an easy one. Life-span is another.
But of course the real question is what makes one country have a high literacy rate, an educated female population, a mean longevity of 8 decades and rising...
Beats me.
I doubt it's genes, or only genes, or mainly genes. More than one Indian tribe in the Americas were on their way to genuine civilization -- had one, in fact -- until the Spaniards got there and pounded them into dust, or some as-yet-unexplained environmental catastrophe did them in.
Go to Guatemala and you just feel pity for poor devils in the countryside, but ... their ancestors were doing serious mathematics and astronomy, creating monumental public buildings, etc. I doubt the genes changed. [The previously-linked-to ferociously-conservative Fred has a good rant on this, refuting those who think it's all genes.]
China was backward for centuries, after being one of the, if not the, most advanced cultures on earth. Now they're coming back, fast ..., while America goes down. I doubt the respective genomes mutated simultaneously.
So ... I fall back on the comfortably-general phrase, "material conditions", refracted through the accidents of history. Roughly speaking, the level of the forces of production, to start with, understanding that these support and shape (but don't mechanically determine) political and social arrangements that in their turn can facilitate their growth, or retardation.
But anyone with even a slight familiarity with history will know that there are peoples/tribes/races/nations -- call them what you will -- which have been at the forefront of human advance, and have then fallen back: the Greeks, the Romans, the Egyptians, the Persians, the Chinese, the Arabs. A genetic change? No.
I'm not really familiar enough with the various theories of why there are such historical cycles to confidently advance or reject any of them.
I just know that 'culture' is important, but that it exists in a matrix of other causes: get overrun by barbarians -- Mongols or Spaniards -- and you appreciate Heraclitus' observation that War is the Father of All Things, or old Bismarck's remark that all the great questions of mankind are settled not by parliamentary majorities, but by blood and iron.
Look at the Australian aborigines, and you can appreciate the extreme difficulty of getting out of the stone age in an extremely unfavorable physical environment. I suppose the same argument could be made for the Amazonian tribes still stuck in hunter-gatherer mode.
Go to Cuba -- a country with a large Black component in its population, incidentally -- and see the stopped up public toilets and decaying buildings -- alongside their really excellent results in education and very low crime rate, compared to other Latin American countries, and you appreciate the influence of the political and economic system.
Compare the theocracy of Iran -- a nation on the way back to cultural greatness -- with the theocracy of Saudi Arabia and you realize religion alone is not an explanation, unless there's something about the Shi'a Sunni divide that I don't understand.
Wish I knew for sure.
What do you think?