CHICAGO - ALTERING a gene in the brain of female worms changed their sexual orientation, United States researchers said on Thursday, making female worms attracted to other females.

The study reinforces the notion that sexual orientation is hard-wired in the brain, said Dr Erik Jorgensen, scientific director of the Brain Institute at the University of Utah.

'They look like girls, but act and think like boys,' Utah researcher Jamie White, who worked on the study published in the journal Current Biology, said in a statement.

Researchers in Dr Jorgensen's lab switched on a gene in female worms that makes the body develop male structures, but they only activated the gene in the brain.

As a result, the female worms still had female bodies, but they behaved like males.

'It suggests sexual behaviour is encoded in our genes' and not caused by extra nerve cells specific to males or females, Dr Jorgensen said in a telephone interview.

Animals such as nematodes, fruit flies and mice share many of the same genes as humans and are often used as models to understand human genetics.

But Dr Jorgensen said the study is not likely to resolve the burning question about the genesis of sexual orientation in humans. 'A human's brain is much more complex than a worm's brain,' he said.

Many scientists think a host of factors such as genetics, hormones and environment may play a role in determining sexual orientation in humans, but this has not been proven.

Dr Jorgensen said the study is interesting because it suggests rather than being caused by extra, sex-specific nerve cells, attraction behaviours are part of the same brain circuit.

The finding was part of a study looking at areas in the worms' brains involved in sexual attraction.