When people join an economy they add supply to the labour market and with the money they earn they add demand to other markets like the one for consumer goods.<p>If we are throwing around Wikipedia entries, I might as well post <a href="http://en.wikipedia.org/wiki/Says_law" rel="nofollow">http://en.wikipedia.org/wiki/Says_law</a> as a counter.<p>Reason has an interesting article about why poor countries are poor (<a href="http://reason.com/archives/2006/03/01/why-poor-countries-are-poor" rel="nofollow">http://reason.com/archives/2006/03/01/why-poor-countries-are...</a>).  The efficiency and honesty of the governments (or the lack of those qualities) play a large role.