To put numbers in perspective, IQ is generally defined as a normal distribution (also called a gaussian curve, bell curve, etc.--it's a "normal" distribution be

To put numbers in perspective, IQ is generally defined as a normal distribution (also called a gaussian curve, bell curve, etc.--it's a "normal" distribution because it's extremely common in nature) with a global average of 100 and a standard deviation of 15 points. What this means in practice is that, roughly, about 2/3 of the population has an IQ in the range 85-115. So, assuming each country also has the same shape of distribution but different averages, a difference of 15 points means that an average person in the "smart" country is smarter than 83% of the people in the other country. An average IQ difference of 30 points would be more like 98%. Which, to me, seems like a pretty extreme difference. Without knowing more about their methodology I don't know if I trust these numbers much. On the other hand, looking at the countries at the lower end, I wouldn't be surprised if a lot of the difference was due to environmental factors such as poor nutrition. Unfortunately, IQ is a field of study that's far too politically charged for there to be much in the way of reputable research.