Eccentric Yoruba (hilarious and throughly eccentric blog) asks, ' I do two strand twists often, could that cause breakage?'
The short answer is yes and no. Some dermatologists have written reports on damage to varying extents from breakage to alopecia (hair loss). Twisting in particular has been studied in more detail. For natural hair twisting means
1. Twists to style hair
2. Twisting during styling (For example using a round brush when blow drying)
What kind of damage is possible?
The kind of damage done includes cuticle chipping, tangling and breakage due to tangling. At low to moderate levels of twisting, the hair is unlikely to break and recovers to its initial state. However at high levels of twisting, the hair cannot fully relax back to its original length and can break (I would quote the number but it is pretty meaningless since you can't measure what force your fingers are using) - (International Journal of Cosmetic Science, p138 , 2005).
Remember though that pretty much everything that we do to hair damages it (shampooing/conditioning, towel drying or combing). The only way not to damage hair is to not touch it at all.
Could there be a benefit?
Nobody has done long term studies on type 4ish African hair, but glancing at fotkis and blogs of long haired naturals, it is clear to see that most if not all use twisting and braiding as their dominant hairstyle ( Sera, Roshini,Kemi, Nikki/Mwedzi, Maestradiva ). This leads me to say that twisting and braiding are perfectly fine provided they are not done tightly.
Other sources used
International Journal of Dermatology,v 38 , i S1, pg 34 - 44
J Am Acad Dermatol, 43(5 Pt 1), pg 814-20, 2000