Unfortunately, there's not a lot of data to answer that question.
And the reason is it's a little bit difficult to determine after the fact
if there was one or two placentas,
someone actually needs to examine the placental tissue.
And that's not routinely done in twin births.
But there are some studies we can look at.
And I'm going to just mention one here from Belgium.
It's a beautiful study of this.
And in this case what they're looking at is cognitive ability.
But it's, I think, although it's a single study, I
think it's fairly illustritive of the findings in this area.
And what they did is, they measured, again, we're
going to measure twins' similarity here in terms of a correlation.
No similarity is zero.
Perfect similarity is one.
And what I've plotted here for three different measures of
general cognitive ability, we can just look at total IQ.
The, the, the pattern is the same for all the three.
Is what's plotted here is the correlation for the monozygotic monochorionic twins.
Those are the 2 3rds of monozygoric, monozygotic twins who
share a placenta, versus the dizygotic twins correlation for IQ.
They always share. They always never share a placenta.
They have two placentas.
And you can see that right, these twins are more similar than those twins.
Is that genetics? Or is it sharing one
placenta, versus having two placentas?
Well, the key observation clearly is the dichorionic monozygotic twins.
Do the dichorionic monozygotic twins look like the green bar?
In which case it's a chorion effect.
It's not genetics. Or does they look like a blue bar?
In which case we conclude that it's genetics and not the placenta.