An intriguing line of research into animals' knowledge of their own mental states considers the performance of animals in situations of cognitive uncertainty.  When primates and dolphins are given a “bailout” response allowing them to avoid making difficult discriminations, they have been shown to choose the bailout option in ways that are very similar to humans (Smith et al. 2003). The fact that animals who have no bailout option and are thus forced to respond to the difficult comparisons do worse than those who have the bailout option but choose to respond to the test has been used to argue for some kind of higher-order self understanding. The original experiments have attracted both philosophical criticism of the second-order interpretation (e.g. Carruthers 2008) and methodological criticism by psychologists (reviewed by Crystal &amp; Foote 2009), although alternative approaches to establishing metacognition in non-linguistic animals may be capable of avoiding these criticisms (Terrace &amp; Son 2009).