A neuroimaging study compares the brains of 50 individuals diagnosed with a major mood disorder characterized by persistent sadness to 50 non-diagnosed individuals. The study finds that, on average, the diagnosed group has a significantly smaller hippocampus. The lead researcher concludes in a press release, 'Our research proves that a shrinking hippocampus is a direct cause of this disorder.' Based on the current understanding of the relationship between brain structure and this condition, critically evaluate the researcher's conclusion. Is the conclusion fully supported by the evidence presented? Explain your reasoning.