• What are the chances that twins are a) two girls, b) two boys, or c) a boy and a girl?

• Try it by tossing two coins.

• Two girls: ¼, two boys: ¼, a girl and a boy: ½.

www.sciensation.org | Ciênsação hands-on experiments are published as Open Educational resources under a Creative Commons Attribution-ShareAlike 4.0 International License.
Maths
age: 15 – 17

A woman gives birth to fraternal twins: what is the probability that they are a) two girls, b) two boys, or c) a boy and a girl?

You might think that the answer is obvious, but surprisingly many people hold on to a misconception: Since there are three possible outcomes listed, each has a probability of 1/3. Even if you explain that the third option actually encompasses two possible outcomes (boy + girl, girl + boy), words alone won't really convince everyone. A brief experiment, on the other hand...

Learning objective

Addressing a common misconception in the perception of probabilities.

Preparation

Discuss with your students if it is more likely for a woman with fraternal twins, i.e. developed from two different eggs, to have a) two girls, b) two boys c) a boy and a girl, or if d) all three possibilities are equally likely.

If you note that some students are tending to option D, make the class do an experiment with coin tosses. Prepare a table on the whiteboard to record your students' results.