Which is more likely, to have twins of the same gender or to have twins of different genders?

Identical twins must be of the same gender. Fraternal twins would have the same frequency of each gender as in overall births, which is approximately 51% male, 49% female. If (to make the maths simpler) we call this half of each, then fraternal twins will be both female a quarter of the time, both male a quarter of the time, and one of each half the time. So for fraternal twins it is 50-50. But overall, there will be more that are of the same gender, once identical twins are included.

As this site shows, the probability of 2 boys and that of 2 girls are the same, just under 1/3; the probabliity of 1 girl and 1 boy is just over 1/3. (The exact values depend on relative frequency of identical and fraternal twins.) This means twins will have the same gender roughly 2/3 of the time.

http://mathforum.org/library/drmath/view/69420.html