The Washington Post this morning carries a story on a DC couple who went on safari in Tanzania and then decided to start an NGO to donate bicycles to give relief to the vast number of female manual porters they encountered. Whether their project fits into the well-populated category of poorly informed good intentions I leave to the readers to judge (although the NGO name is the cringe-inducing Pets Providing Pedals, since one of the couple is a professional dog groomer). Every visitor to Africa is struck by the huge amount of human porterage going on, usually by women and girls. The stereotypical image of an African girl walking long distances with a large load balanced on her head is not just a stereotype. But the Pets Providing Pedals project raises a different question -- why aren't bicycles already used a lot more already? Or carts drawn by draft animals? Or cars or trucks?
A standard economist's answer could suffice, although it hardly lessons the tragedy of the women condemned for life to porterage. You substitute capital (trucks, bicycles, carts) for labor (head porterage) when labor is scarce. You substitute labor for capital when capital is scarce and unskilled labor is abundant. Guess which one applies to most African countries.
A sustainable alternative to women being used as draft animals probably requires something that vastly increases the demand for unskilled labor and makes it more expensive ("sweatshops" look positively attractive by comparison). Of course, there are also these little tiny issues about women's rights and gender equality -- but that too could respond to economic forces that gives women many more viable alternatives.