However, there is also considerable interaction between the first and second language and so a straightforward 'regression pattern' cannot be observed. Also, parallels in noun and verb phrase morphology could be present because of the nature of the tests or because of avoidance by the participants. In a follow up 2010 article, Keijzer suggests that the regression hypothesis may be more applicable to morphology than to syntax. Citing the studies on the regression hypothesis that have been done, Yukawa says that the results have been contradictory. It is possible that attrition is a case-by-case situation depending on a number of variables (age, proficiency,& literacy, the similarities between the L1 and L2, and whether the L1 or the L2 is attriting). The threshold hypothesis states that there may be a level of proficiency that once attained, enables the attriting language to remain stable. Factors Age effect 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 of 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. Attrition of a first language does not guarantee an advantage in learning a second language. Attriters are outperformed by native speakers of the second language in proficiency. A 2009 study tested the Swedish proficiency of Swedish speakers who had attrited knowledge of Spanish. These participants did show almost but not quite native-like proficiency when compared to native Swedish speakers, and they did not show an advantage when compared with bilingual Swedish-Spanish speakers. On the other hand, L1 attrition may also occur if the overall effort to maintain the first language is insufficient while exposed to a dominant L2