Children are more susceptible to (first) language attrition than adults. Research shows an age effect around the ages of 8 through 13. Before this time period, a first language can attrite under certain circumstances, the most prominent being a sudden decline in exposure to the first language. Various case studies show that children who emigrate before puberty and have little to no exposure to their first language end up losing the first language. In 2009, a study compared two groups of Swedish-speaking groups: native Swedish speakers and Korean international adoptees who were at risk for losing their Korean. Of the Korean adoptees, those who were adopted the earliest essentially lost their Korean and those adopted later still retained some of it, although it was primarily their comprehension of Korean that was spared. A 2007 study looked at Korean adoptees in France and found that they performed on par with native French speakers in French proficiency and Korean.