Mood lability refers to changes in mood that are notable for their size, frequency and speed so someone with labile mood is prone to sudden, frequent and major changes in mood. Researchers from the Institute of Psychiatry in London studied the levels of mood lability in 5326 young people aged 8-19. They found that mood lability occured in 5% of the sample. It was strongly associated with a wide range of mental-health problems and was linked to significant impairment even in the absence of psychiatric disorders. Mood lability was found to be a risk factor for externalizing disorders (bad behaviour) and internalizing ones such as depression and anxiety.
Stringaris, A. and Goodman, R. - Mood lability and psychopathology in youth Psychological Medicine August 2009, 39(8), 1237-1245