Why do the Yoruba have the highest number of twins in the world?

Why do the Yoruba have the highest number of twins in the world?
…. Because we love twins. Igbó-Ọrà in Ọ̀yọ́ State is special!

Èjìrẹ́ ará Ìṣokùn ọmọ Edunjọbí
M̀bá be’jirẹ o, inu mi a dun o
Ẹ̀rù o bàmí o, rárá o, rárá o
Àyà o fo mí o, rárá o, rárá o
Láti be’jirẹ o, Èjìrẹ́ dára,
Mo l’épo ní 'lé, mo lẹ́wà l’ọdọ̀n o
Táyélolú ijó ooo, ijó; Èjìrẹ́ ijó ooo, ijó
Bi mba be’jirẹ, mba yọ dandan,
Ijó; Èjìrẹ́ ijó ooo, ijó
O wọlé alákisà, o sọ alákisà da aláṣọ

- Song by King Sunny Ade (KSA)

Translation....

Twins, native of Ìṣokùn town; you, children of Edunjọbí
Had I given birth to twins, I would have been happy
I am not afraid of having twins at all
I am not scared of having twins at all
Twins are beautiful
I have palm oil at home and and beans in the yard
Táyélolú it is time for dance, yes it is time
Èjìrẹ́ (Kẹ́hìndé) it is time for dance, yes it is time
Had I given birth to twins, I would have been very very happy.
It is dance time, my twins, it is time to dance
Twins who turn around misfortunes - turning the wearer of ragged clothes to a prosperous person.

Note: Beans cooked with palm oil is the favourite food of twins in Yorubaland.