A recent study* found that those who sit upright reported higher self-esteem, more arousal, better mood and lower fear when compared to slumped sitters. Sittting upright also increased the rate of speech and reduced self-focus. It concluded therefore that sitting upright can be used as a strategy to help build resilience to stress.
So not only do you reduce your risk of back pain by sitting in a healthy upright posture, you can also increase your ability to deal with stress. Being more alert and in a more positive mood can also greatly increase your wellbeing and productivity levels. Good posture is therefore extremely important for your health in many ways that you may not immediately think of.
*Do Slumped and Upright Postures Affect Stress Responses? A randomized trial. Health psychology 2014. S Nair, M Sagar, J Sollers, III, N Consedine, E Broadbent