One quantity of crucial importance for understanding evolutionary history is the allele frequency spectrum. I will explore its mathematical properties and will report data mined to overcome ascertainment biases and obtain accurate estimates. Analysis of these data shows that the ancestors of East Asians and North Europeans shared the same population bottleneck dispersing out of Africa but that both also experienced a more recent bottleneck. Contrasting chromosome X with the rest of the genome further shows that around the time of the dispersal out of Africa chromosome X experienced a more extreme reduction in population size, which points to a sex-biased demographic event during that epoch of human genetic history.