Spain is not unique in that sense. All the countries in the world have an informal sector that is not accounted for in statistics. According to a paper from the World Bank [1] the size of the shadow economy in Spain in 2007 was 22.2% of the GDP, whilst the average for the OECD countries was 16.6%. In the UK the figure was 12.2%.

So while it is true that the informal sector is somewhat larger in Spain than in other countries, the difference does not seem to be enough to explain such high unemployment rates.

[1] http://www-wds.worldbank.org/servlet/WDSContentServer/WDSP/I...