There are four languages of the Swiss. German, Italian, French, and Romish. Their cultural history is from the the Alps, and the alpine environment is often cited as an important factor in the formation of the Swiss national character. The "Swiss illness", the condition of Swiss mercenaries pining for their mountainous native home, became prototypical of the medical condition of homesickness described in the 17th century. With more than 20% of the population resident aliens, Switzerland has one of the highest ratios of non-naturalized inhabitants in Europe.