Last week's Science section of the New York Times had an article about variations in sex ratios by latitude, based on an article in Biological Letters. Essentially, the upshot is that people in Africa are less likely than people in Europe and Asia to have a boy (even after excluding data that might be affected by sex selective abortions). Garenne wrote about sex ratios in Africa in Human Biology in 2002 and found broadly similar results. You can read the NYT article here.