Only women possess the ability to give birth and to nurse new human beings.

ECONOMIST: Unequal opportunities

Children appear normal at birth, but gradually lose the ability to walk, talk and eat.

WSJ: Families Push for New Ways to Research Rare Diseases

"My guess is that since economic times have worsened, people are concerned about their ability to raise larger families and are opting for more permanent birth control, " Ross said.

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But it is still debated whether humans are born with an innate ability to process speech or whether this is something acquired through learning after birth.

BBC: Babies can hear syllables in the womb, says research