Sex can be expensive and risky. It takes away time and drains precious nutrient resources. And each sexual act runs the risk of messing up carefully constructed genetic blueprints. So, why do people still do it?

The answer may seem obvious, but it isn't so clear to biologists who consider that despite a logical alternative--asexual reproduction by simple cloning without the help of a partner--sex is preferred in the wild.

Asexuality, which happened to come first, is seen in plants that send out underground runners and flatworms which, when severed in half, grow a new head on one half and a new tail on the other half. In fact, some microbes and fungi lean this way, too, and have been since life began.

Scientists don't even know how sex began. But they have speculated for a long time that organisms prefer sex specifically because of the risk. As the thinking goes, the slight rearrangement of genes production through reproduction may help organisms adapt more easily to a stressful or changing environment. However, belief is not the basis of conclusions.

A new study using genetically modified yeast helps to setting the question of if sex is indeed beneficial.An experiment set a strain of yeast that reproduces sexually against a modified, asexual version of the same strain. According to Matthew Goddard of the University of Auckland, each grew and reproduced at the same rate.

When he and his colleagues raised the stakes by providing each subject less food, those engaging in the ultimate act still managed a growth rate of 94 percent, whereas the asexual strain only managed to reach 80 percent. This leads us to believe that sexual organisms are fit to survive. These finding are detailed in the March 31 issue of Nature.

There are weaknesses in the study. For one, it does not explain why sexual organisms are hardier in stressful condition, or why the burden of sexual reproduction differs so greatly between males and females. Not only do females spend more time developing and raising children, but, in most cases, their sex cells are significantly more expensive to make compared to a male's.

"We are still far from a definitive answer to the question of why sexual reproduction is so common," said Rolf Hoekstra of Wageningen University in a review he wrote of the study.