Submitted on: 21 May 2006.
Revised on: 23 August 2006.
Accepted on: 23 September 2006.
___________________JOURNAL OF BIOSOCIAL SCIENCE---CAMBRIDGE UNIV PRESS
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Submitted on: 23 January 2011.
Revised on: 14 March 2011.
Accepted on: 09 April 2011.
___________________JOURNAL OF BIOSOCIAL SCIENCE---CAMBRIDGE UNIV PRESS
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