I've heard it's 33% during the Qing dinasty. 4% during cultural revolution. Now it's climbing up steadily to 11%.
Where can I see the table of those actually tracking those numbers?
What were the causes?
Han dynasty (206 BC–220 AD)
Tang Dynasty 618 – 907 AD GDP per capita:$480, 58% of world GDP
Song Dynasty 960-1279 AD GDP per capita:US$2,280, 80% of the world’s GDP
Yuan Dynasty 1271-1368 A estimated to account for about 30% -35% GDP
Ming Dynasty 1368–1644 AD GDP per capita:US$600, 55% of world GDP
Qing Dynasty 1644-1922 AD GDP per capita:US$600 Qing Dynasty accounted for 35% -10% of the World GDP
People’s Republic of China 1949- GDP per capita:US$5414, 9.48 percent of the world economy. See also Wikipedia for this period.
There is also another article at Wikipedia: Economic history of China before 1912
Maybe you can find hint to other sources there.
What is China has changed over time, as have the number of people in China, as has China's level of per capita output (GDP prior to capitalism is an anachronistic imposition if measured in a current value expression), as has the number of people not-in-China, as have the level of per capita output in places not-China.
The assumption that China's proportion of world output would remain static is a less tenable hypothesis than that all proportions of long lasting cultural nexuses of output would vary.
What you're really asking is "Was Needham right?" http://en.wikipedia.org/wiki/Joseph_Needham#The_Needham_Question
And there's a great deal of literature about prefigurative forms of capitalism in China out there. I'd suggest you start by reading the review articles on the Needham Question, because scholarly discontent with the failure of China, a large high productivity advanced feudal society*1 to cement the prefigurative forms of capitalism into actual capitalism continues.
*1 Marxist use in relation to circulation of prestige, status, and direct extraction techniques; not a claim of infeudation.
Well, at least the drop during the "Cultural Revolution" is easily explained. Setting hordes of young, largely semi-literate, thugs to torture and maim the productive and educated members of society will do wonders for the economy. Here is a long quote from wikipedia: