Intersex Twins 

  The Economist  and  Time Magazine  recently published interesting articles on a new type of twins.  Apparently some twins are neither identical nor fraternal, but are `semi-identical'.  That is, one twin is male and the other `intersex' (both male and female).  You can read a short discussion on the biology in the articles, which also note that it’s unknown how common this type of twins is. More to worry about for believers in twin studies (for other problems, see  this  earlier post).