Fraternal Twins

About 2/3?

 

Chances

Girl - Girl
Girl - Boy
Boy - Girl
Boy - Boy


Odds are 75% at least one will be a girl.

Rob

 

I'd argue that your options 2 and 3 are the same; thus, 2 out of 3....

 

Although, I do accept that it also depends on which came out first, so I understand 75%....

 

Now I know Dr. Cassidy as a very precise person on the BB - he reads things exactly as they are written.

The question as raised in the OP was, "What is the chance that one of the twins will be a girl?"

As he noted, there is a 50% chance that only one will be a girl.

Rob

 

With al due respect to Dr. Cassidy and you -- incorrect. First, the opening post did not specify "only" one. Second, in accordance with the wording of the opening post, girl/girl is a valid option, raising the odds to 66% instead of 50%. If the opening post had specified "only" one is a girl, then the options would be limited to boy/boy and boy/girl, resulting in 50%. If the opening post had specified the birth order, then Deacon's 75% would be valid. But a strict reading of the opening post leaves only the 2 out of 3 option.

 

Well...my apologies. The question is more complex than I originally thought.

On the one hand, with twin comparisons, the answer is 66%. However, if one starts using truth tables, you get:
B G
B F T
G T T
(B-boy, G-girl, T-true, F-false)
And thus, 75%.

Alternatively, upon birth of the first child, if it's a boy, there's a 50% chance the next child will be a girl. If the first child is a girl, then we've reached 100% probability. The average of these two alternatives is also 75%.

You could make it even more difficult, and throw in the probability that 40% of twins are boy/girl, leaving 30% equally for boy/boy and girl/girl (except that statistically, boy/boy twins miscarry more than girl/girl, so that actually increases the odds of a girl child....)

In the end analysis, I should probably apologize to Deacon and go with the 75%....

 

"Statistics, my dear boy. Statistics."

But here's more than you wanted to know (the title isn't entirely accurate, as there's more info).

24 Interesting Fraternal Twins Gender Statistics - HRF