My neighbors insist that there are methods by which a couple can increase their chances of having a child of a particular sex, and that these methods have to do with different characteristics of male and female sperm cells. The believe that the habits and diet of the man can also affect the ratio of male and female sperm cells he produces. They also said that certain couples are more likely to have children of one sex, and that it is more common to see families with several children of the same sex than would be expected if the odds for each child were 50/50.

I've done some research and I'm not finding good evidence for any of this. Everything I'm finding indicates that meiosis in men always produces half male sperm, half female sperm, that there is no difference between them in morphology or behavior, and therefore the odds of having a boy or a girl really are 50/50. Am I right about this or is there something I am missing?

Any proposed behavioral changes in parents do not have a have an affect on the gender of offspring. 

Families with large numbers of male or female offspring result from the "rule of large numbers." If you flip a coin enough times you  will eventually get heads five times in a row simply by chance.

There are known ways of influencing the sex of a child. Female sperm are slightly heavier than male sperm due to the extra mass of the x chromosome and will tend to sink to the bottom of a test tube when centrifuged. This allows some parents using in vitro fertilization to select the sex of a child with greater accuracy than chance. 

Perhaps the most effective (and ethically dubious) method used by parents is selective abortion where only children of the desired gender are brought to term.

Third time lucky (my other attempts have all failed to post).
Males do indeed produce equal numbers of X and Y sperm. Your neighbour may be using a variant of the Shettles method that has been around since the 1970s, which works on the assumption that X and Y sperm travel at different speeds, have different longevity and are sensitive to different levels of acidity. There is little support for this theory. X and Y sperm swim at the same average speeds, although their behaviour is apparently different in variable flow conditions, probably due to the difference in mass (http://www.ncbi.nlm.nih.gov/pubmed/6541168).


There is some evidence that females with high energy (or possibly nutirent) intake prior to conception are slightly more likely to have male children (http://rspb.royalsocietypublishing.org/ … /1661.full). However, the difference is only a few percent and I can find no evidence to suggest that male nutrition or habits play any role at all.