The author used GDP per capita as noted in the comments on the page. Why not use a less clickbait title like "why is UK's GDP per capita lower than US?", his piece would have sounded less intellectually dishonest.
I do not think that solves the question. What is the reasoning behind the assumption that GDP/capita in the UK should be equal to that of the US?
And why should it not be equal to say, that of Italy instead?
The author is assuming a certain "natural order of things" and placing the UK a few ranks higher than it actually stands.
The author is assuming a certain "natural order of things" and placing the UK a few ranks higher than it actually stands.