Today's intermittently diurnal tidbit comes from a book I doubt any of you will want to read: Kinesiology - The Skeletal System and Muscle Function by the improbably named Joseph Muscolino. Nevertheless, this (somewhat paraphrased) extract might be of interest if you're either a habitual carrier of laptops or a female lady of the feminine gender.
Carrying a purse or any type of shoulder bag is an extremely common habit that can lead to postural distortion. Although the weight of the bag is important it is not the most important aspect. Even carrying an empty shoulder bag can cause the muscles that elevate the shoulder girdle to spasm and so distort the posture. The reason is that the natural slope of the shoulder is such that a bag would fall off. To prevent that from occurring, a person will isometrically contract musculature to elevate the scapula. This long-standing isometric contraction eventually leads to a chronic postural problem regardless of the weight of the bag (although greater weight means the muscles will have to contract more forcefully). Wearing a bag that has a strap that goes over the opposite shoulder and across the body is healthier... Just in case there's any doubt what that said: you have to hunch your shoulder to stop a bag, even an empty one, falling off and that messes up your posture eventually. Putting the strap over your head will help a bit.