Gay men more likely to have more older brothers, according to new research Gay mane are more likely to have more older brothers than straight men, new research suggests. THE more older male siblings a man has, the greater chance he will be gay, according to a series of studies. Scientists say the phenomenon cannot be explained by the youngest boy being babied and mollycoddled or other differences in the way they are brought up. Instead, it has a biological basis, claim researchers. They believe the immune response a woman mounts against having a male baby in her womb increases with each son, raising the odds of "feminising" the foetus's developing brain. In one study, Canadian scientists collected data from almost 1,000 men, including some who had been adopted or brought up with stepbrothers and stepsisters. This revealed that having lots of brothers raised a man's odds of being gay - but only if they were blood brothers. Stepbrothers did not have an effect, the research by Brock University in Ontario found. However, biological brothers who were brought up separately did. This suggests that the link can be explained by nature rather than nurture, New Scientist reports. Each older brother raised the odds that a man was homosexual by a third. This means that if a first-born son has a 3 per cent chance of being gay, the figure for the second son will be 4 per cent. By the fourth son, the odds will have more than doubled. It is thought that carrying a male baby in the womb triggers an immune response in the mother, creating antibodies that attack part of the unborn child's brain linked to sexual orientation. This response gets stronger the more boys a woman carries, raising the odds of homosexuality. Genetics and exposure to hormones in the womb are also likely to be important, while some argue that upbringing plays a role in sexuality. Female fetuses do not provoke the same reaction and no link has been found between having lots of sisters and lesbianism. This week's New Scientist reviews other studies that link physical and psychological traits to a person's place in the order of siblings - or parity. Studies show eldest children to be more cautious but also more intelligent. Middle children tend to be more sociable while youngest children are less likely to suffer from allergies and asthma. The science magazine states: "Not all younger siblings will be spoilt and allergy-free, not all middle-borns will be social butterflies and not all older siblings will be tall, intelligent, responsible leaders, but our place in parity provides fascinating insights into the complexity that makes each of us unique."