Bearing these remarks in mind, decide what is being explained and what is being argued for in the following passage. Is the argument convincing to you? Since 1980 [the number of human twin births has]...

Bearing these remarks in mind, decide what is being explained and what is being argued for in the following passage. Is the argument convincing to you? Since 1980 [the number of human twin births has] been increasing – they are up by 42 per cent since that date throughout the developed world. For every 1,000 births in the developed world 29 are now twin births. This fi gure is likely to continue to increase because many of the extra twin births are caused by fertility treatments which result in the release of more than one egg during ovulation. This is also why the whole increase in twin births is taken up by fraternal twins who come from separate eggs. The rate of identical twins, from division of the same egg, has remained constant at 4 for every 1,000 births. So, while the number of twin births is set to increase even further in the short term as more women seek fertility treatments, there will be a limit to the number of extra twin births because, fi rst, not all will seek this treatment and, second, not all women who do will have twins.

At what point the fi gure will stabilise we do not yet know, but we can rest assured that the majority of births will remain as single births. (‘Twin sets’, New Scientist, 18 November 2000, p. 121)