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Police officials in Cancun said local police chief of staff Wilfrido Flores, 56, and his bodyguard were shot dead in their car in a busy avenue in the Caribbean resort, one of Mexico's biggest tourist pulls, just before midnight.

However they played down links to a spate of similar killings in cities like Acapulco and Tijuana in a brutal war among drugs cartels and security forces.

"They were fired at half an hour after leaving the office. We don't know how many or who they were. (Flores) had 15 bullet wounds, mainly in his head and neck. A lot of his face was destroyed," said police spokesman Oscar Meza.

"It surprised us. We can't see a motive. He works in an office planning public security. He's not on operations and has nothing to do with investigating drug cartels," he said.

Last weekend alone, kidnappers killed four policemen in the Pacific coast state of Guerrero -- gagging and shooting three and beheading another -- and seven civilian murders were reported.

Last week, police in Tijuana said they had found the severed heads of three police officers and a fourth man.

Drug violence and attacks on police have risen in the run-up to the July 2 election, which has leftist Andres Manuel Lopez Obrador and conservative Felipe Calderon in a close race for the presidency.

"Of course, they are not creating any problem for the election," President Vicente Fox's spokesman Ruben Aguilar said.

The violence flared up after Fox launched a war on drug cartels early last year.

Police killings are rarer in Cancun, although the resort has seen a handful of gory civilian murders in recent months. Flores was a former army colonel.

Past research by Dr. Anthony F. Bogaert of Brock University in St. Catherines, Ontario and colleagues has shown that the more older brothers a man has, the more likely he is to be gay. But it has not been clear if this is a prenatal effect or a psychosocial effect, related to growing up with older male siblings.

To investigate, Bogaert studied 944 gay and straight men, including several who were raised with adopted, half- or step-siblings or were themselves adopted. He reasoned that if the relationship between having older male siblings and homosexuality was due to family environment or child-rearing practices, it would be seen whether or not a man's older brothers were biological or adopted.

Bogaert found that the link between having older brothers and homosexuality was present only if the siblings were biologically related -- this relationship was seen between biological brothers who were not raised together. The amount of time that a man was reared with older brothers had no association with sexual orientation.

"These results support a prenatal origin to sexual orientation development in men and indicate that the fraternal birth-order effect is probably the result of a maternal 'memory' for male gestations or births," Bogaert writes in his report in PNAS Early Edition.

A woman's body may see a male fetus as "foreign," Bogaert explains, and her immune response to subsequent male fetuses may grow progressively stronger.

"If this immune theory were correct, then the link between the mother's immune reaction and the child's future sexual orientation would probably be some effect of maternal anti-male antibodies on the sexual differentiation of the brain," he suggests.

Other lines of research also support the sexual orientation-maternal immune response link, he notes.

SOURCE: PNAS Early Edition, June 26, 2006.