They put it together by assessing 23 criteria, including foreign wars, internal conflicts, respect for human rights, the number of murders, the number of people in jail, the arms trade, and degrees of democracy (Guardian).
You can see a world map of peace at the Vision of Humanity website, and also take a look at country rankings for 2009, as well as earlier years.
New Zealand came top this year. Hmm, New Zealand is a pretty non-religious country. In fact, if you eyeball the rankings, the top few countries are all pretty non-religious.
What I've done in the figures here is to take data from the World Values Survey on the percentage of people in each country who say they are a committed atheist, and also on the percentage of people who say that they go to a religious service at least once a month.
Then I split the sample into two equal groups, based on their score on the Global Peace Index. The ones in the 'Peaceful' group are countries with a GPI score less than 1.8.
Sure enough, peaceful countries have more atheists and fewer regular worshippers. The difference is highly statistically significant (P=0.001 or less) - in other words it's real, not just a chance finding.
Now, there are several possible reasons for this. It could be that people living in turbulent countries turn to religion, or it could be that religion is not a good way to structure modern society. Or it could be that some other factor or combination of factors (democracy? free speech? education? government welfare?) generates citizens who are both peaceful and non-religious.
Whatever, it's another blow to the idea that secularization leads to social meltdown. Atheist countries are, in fact more peaceful.
Follow up post: Of Secularism and Correlations (also, what the word 'secular' means).
This work by Tom Rees is licensed under a Creative Commons Attribution-Share Alike 2.0 UK: England & Wales License.