A. Functions of the body clock.

　　B. The "night owl" phenomenon.

　　C. Human beings' sleep behaviour.

　　D. The school schedule of "early birds".

　　D

　　Women are friendly. But men are more competitive. Why? Researchers have found it's all down to the hormone oxytocin (荷尔蒙催生素). Although known as the love hormone, it affects the sexes differently.

　　"Women tend to be social in their behavior. They often share with others. But men tend to be competitive. They are trying to improve their social status," said Professor Ryan.

　　Generally, people believe that the hormone oxytocin is let out in our body in various social situations and our body creates a large amount of it during positive social interactions (互动) such as falling in love or giving birth.

　　But in a previous experiment Professor Ryan found that the hormone is also let out in our body during negative social interactions such as envy.

　　Further researches showed that in men the hormone oxytocin improves the ability to recognize competitive relationships, but in women it raises the ability to recognize friendship.

　　Professor Ryan's recent experiment used 62 men and women aged 20 to 37. Half of the participants（参与者）received oxytocin. The other half received placebo (安慰剂).

　　After a week, the two groups switched with participants. They went through the same procedure with the other material.

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