Your mother was right, you should sit up straight. A new study has been done that examined how your posture when sitting affects your self-image.

Read about the posture study here.

Brinol et al. (2009) divided 71 students into four groups, then fed them a cover story about why they had to adopt two different poses: half slouching, the other half sitting up straight, chests puffed out. These two groups were then split again, and half were asked to write down three positive personal traits, the other half three negative personal traits.
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The results showed that people who had been sitting up straight were much more likely to believe the positive things they’d been writing about themselves, whereas those who were slouching weren’t so sure. Meanwhile a doubtful posture had very little effect on the half who were thinking negatively about themselves.