How large should my study be so that I can detect an altered sex ratio?

The world literature on manipulations purported to alter the gender of offspring is a quagmire full of reports presenting inadequate data and contradictory conclusions. Many are due to inappropriate statistical design or analysis. The authors present simple, nontechnical reference tables for determination of the minimum sample size necessary to detect a change in sex ratio (1) from a theoretical value, and (2) between two experimental populations. Simple methods for analyzing the results of gender manipulation experiments are described. Technical details are included in a separate section.