Identical twins are a perfect test case for theories of personality development.If a theory can’t explain the differences between identical twins，then it cannot explain environmental effects on per-sonality.Even identical twins brought up in the same home have different personalities.Take Ladan and Laleh Bijani from Iran.They were identical twins who had spent their entire 29years joined at the head.And yet，Ladan，the more outspoken of the pair，told journalists，’We are two completely separate individuals.we have different world views，We have different lifestyles，we think very differently about issues.’Why did Ladan and Laleh have different person-alities？
Self-organized systems in insects can provide us with some ideas.A colony of ants，for exam-ple，can be seen as a self-organized system.No supervisor tells the ants what to do，and yet all the jobs get done.The system works in such a way that if one ant carries out a particular job，it be-comes less likely that another ant will attempt that job because it no longer needs doing.The result is what economists call ’division of labor.’
Self-organization also produces division of labor in human groups.Each individual looks for something to specialize in，his or her own suitable position in the group.If one position is occu-pied，the individual will seek another.This process increases the differences even between identical twins，because once they’ve chosen different specialities，a circular mechanism causes small initial differences between them to widen.
Although identical twins look very much alike，people who know them well will distinguish between them.They might ，for example，address more questions and comments to one twin than the other-perhaps by chance first.But the consequence is that the twin who is addressed more often will do more talking than the other twin，which will cause people who know them to address still more of their questions and comments to that twm.The result，over time，will be one outspo-ken twin and one quieter one-like Ladan and Laleh Bijani.

It was the district sports meet My foot still hadn’t healed（痊愈）from a（n）（36）injury.I
had（37）whether or not I should attend the meet But there I was，（38）for the 3，000-meter run.
’Ready...set...”The gun popped and we were off.The other girls rushed （39）me.I felt （40）as I fell farther and farther behind.
’Hooray!’shouted the crowd.It was the loudest （41）I had ever heard at a meet.The first-place runner was two laps（圈）ahead of me when she crossed the finish line.
’Maybe I should （42），’I thought as I moved on.（43），I decided to keep going.During the last two laps，I ran （44）and decided not t0（45）in track next year.It wouldn’t be worth it，（46）my foot did heal.
When I finished，I heard a cheer—（47）than the one I’d heard earlier.I turned around and （48）.，the boys were preparing for their race.’They must be cheering for the boys.’
I was leaving （49）several girls came up to me.’Wow，you’ve got courage!’one of them told me.
’Courage？I just（50）a race!’I thought.
’I would have given up on the first lap，’said another girl.’We were cheering for you.Did you hear us？’
Suddenly，I regained （51）.I decided t0（52）.track next year.I realized strength and courage aren’t always （53）in medals and victories，but in the （54）we overcome（战胜）.The strongest people are not always the people who win，（55）the people who don’t give up when they lose.