Identical and Fraternal Twins. In Exercises 25–28, use the data in the following table. Instead of summarizing observed results, the entries reflect the actual probabilities based on births of twins (based on data from the Northern California Twin Registry and the article “Bayesians, Frequentists, and Scientists” by Bradley Efron, Journal of the American Statistical Association, Vol. 100, No. 469). Identical twins come from a single egg that splits into two embryos, and fraternal twins are from separate fertilized eggs.

The table entries reflect the principle that among sets of twins, 1/3 are identical and 2/3 are fraternal. Also, identical twins must be of the

same sex and the sexes are equally likely (approximately), and sexes of fraternal twins are equally likely.

Fraternal Twins If a pregnant woman is told that she will give birth to fraternal twins, what is the probability that she will give birth to two girls?

Solution 28BSC

= 0.25