Research that failed to support the existence of the CPH

Well known research that failed to support the existence of the Critical Period Hypothesis is the one by Snow and Hoefnagel-Hohle (1978). First they were supporting the correlation between the first language acquisition and the cerebral lateralization, or in other words they believed in the existence of the Critical Period Hypothesis. They stated that the first language acquisition must occur before the cerebral lateralization is complete, and strongly believed in the prediction of the Critical Period Hypothesis that the second language acquisition will be native-likeness as the first language acquisition only if it occurs before the period of puberty. Hence, Snow and Hoefnagel-Hohle made a research in which they were testing the naturalistic acquisition of Dutch by English speakers. The individuals were divided into 3 different age groups, testing during a period of one year. The 12-15 year old individuals and the adult learners were acquiring the Dutch faster than the other learners. At the end of the research 8-10 and 12-15 year old individuals achieved the language optimum, while 3-5 year old individuals showed lowest results. The final output fails to support the Critical Period Hypothesis or in other words the critical period extending from age 2 to age 12 does not exist.  

This research pointed out on two new facts: the learner who has mastered the first language will easily acquire the second language and the constitution of cerebral dominance eases the second language acquisition. Some of the linguistic experts emphasized that the results of this research are not valid because the age differences are not considered, also putting an accent on the individual differences such as complete control of pronunciation, grammar, and vocabulary, considering as one of the limitations that caused this research to fail.