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This is the VOA Special English Health Report.

A few months ago, government researchers released the latest American Time Use Survey. It included the average time per day that Americans spent providing child care between 2006 and 2010.

Among adults living with children under age six, women spent one hour and six minutes a day providing physical care. This is care like bathing or feeding a child. Men spent 26 minutes.

Yet a new study suggests that men are biologically designed to care for their children. It confirmed that testosterone levels drop after men become fathers.

Testosterone is the main sex hormone responsible for the changes when a boy develops into a man. Women also produce testosterone but in much lower amounts.

Earlier studies showed that fathers have lower levels of testosterone than childless men. Christopher Kuzawa at Northwestern University in Evanston, Illinois, says there are at least two possible explanations.

CHRISTOPHER KUZAWA: "Is it that fatherhood reduces your testosterone? Or do men with low testosterone to begin with, are they more likely to become fathers? And so what we did is, we followed men through time and measured their hormones before and after they became fathers."

Professor Kuzawa is a biological anthropologist. He and other researchers used information from a long-term study of men in the Philippines. That study measured their testosterone levels in 2005 and 2009.

Some of the men were childless; others became fathers during that time.

CHRISTOPHER KUZAWA: "And those are the men where we see the largest decline in testosterone."

The study found that testosterone levels dropped by about half immediately after the birth of a child, then rose some. Men who were active in child care produced less than those who were not active.

Testosterone levels may drop in fathers to get them to pay more attention to parenting than to reproducing. But Professor Kuzawa says there are other good reasons as well.

CHRISTOPHER KUZAWA: "Having high levels of testosterone can increase your risk for diseases like prostate cancer [and] testicular cancer. Also, testosterone can suppress the immune system so that you're less capable of fending off pathogens."

The study is in the Proceedings of the National Academy of Sciences.

Studies have shown a similar drop in other mammals and in birds after mating periods to allow for care giving. Animal studies have shown that the greater the involvement of males, the lower their testosterone levels.

We asked Thomas McDade, another author of the study, if testosterone levels also drop in men who adopt children. He says that is an interesting question and the "next obvious thing to study."

And that's the VOA Special English Health Report, written by Caty Weaver. I'm Jim Tedder.

testosterone: male sex hormone that is produced in the testes and responsible for typical male sexual characteristics 睪酮，睪丸素（男性荷爾蒙的一種）

anthropology: the scientific study of the origin, the behavior, and the physical, social, and cultural development of humans 人類學

prostate: a gland in male mammals that surrounds the neck of the bladder and urethra and secretes a liquid constituent of the semen 前列腺

testicular: of or relating to a testicle or testis 睪丸的；睪丸狀的

pathogens: an agent that causes disease, especially a living microorganism such as a bacterium or fungus 病原體；病原菌；致病菌

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（來源：VOA 編輯：張若瓊）