Believe it or not, homosexuality is not only a matter of choice but also something very well printed on people’s brains, thus shedding new light in understanding homosexuality, new research published in the Proceedings of the US National Academy of Sciences shows.

Scientists at the Stockholm Brain Institute in Sweden led by Dr. Ivanka Savic-Berglund, a neuroscientist of the Karolinska Institute performed magnetic resonance imagery (MRI) scans and positron emission tomography (PET) scans on 90 volunteers’ brains (50 heterosexuals and 40 homosexuals).

The study surprisingly found strong similarities between gay men and straight women on the one hand, and lesbians and straight men, on the other.

More exactly, MRI scans showed that gay men had symmetrical brains like those of straight women, and lesbians had slightly asymmetrical brains like those of the straight men. In other words, the cerebrum (the largest part of the brains responsible for thought, sensory processing, movement, and planning) was larger on the right hemisphere of the brains than on the left in lesbians and straight men. The right hemisphere is responsible for holistic reasoning on many levels, including language.

Next, the scientists used PET scans to measure blood flow in the amygdalae (an almond-shaped structure inside each brain hemisphere), which processes emotion, mood and anxiety. The volunteers were scanned while resting and smelling unscented air.

The scans showed that lesbians seemed to react more like straight men, while gay men were more like straight women. More exactly, the blood flowed to areas involved in fear and anxiety in the case of gay men and straight women, while, in the case of straight men and lesbians, the blood flowed to areas linked to aggression.

What this study seem to highlight is that sexual orientation might be something you are born with and not something that you choose later in life, Sandra Witelson, a neuroscientist at the Michael G. DeGroote School of Medicine at McMaster University in Ontario, Canada. She said that amygdalae develops very early suggesting they are primarily genetically determined and not a “product of learning or environmental influences,” National Geographic reports.

However, she cautioned that the environment could also play a role in defining people’s sexual orientation, giving as example heterosexual men in prison who engage in homosexual behavior although they are not homosexual, but because they have no other choice.

This is not the first study suggesting that people’s sexual orientation is influenced by biological factors. Neurobiologist Simon LeVay reported ten years ago that a key area of the hypothalamus, a brain structure linked to sexual behavior, was smaller in homosexual men than in heterosexual men.