People with Williams Syndrome treat strangers as their new best friends. Now the condition is giving clues to our evolutionary past – and what makes us human.

Imagine walking down the street and feeling an overwhelming love and warmth for every single person that you met. That is a familiar experience for people with Williams Syndrome (WS), a rare genetic condition that affects approximately 1 in 7,500 individuals.

People with WS, often dubbed the 'opposite of autism', have an innate desire to hug and befriend total strangers. They are extremely affectionate, empathetic, talkative and gregarious. They treat everyone they meet as their new best friend, yet there is a downside to being so friendly. Individuals often struggle to retain close friendships and are prone to isolation and loneliness.

People with WS are also sometimes too open and trusting towards strangers, not realising when they are in danger, leaving them vulnerable to abuse and bullying.

"It's very easy for someone to fool a person with Williams Syndrome and take advantage of them, because they are so trusting," says Alysson Muotri, a professor of paediatrics and cellular and molecular medicine at the University of California, San Diego (UCSD). "They give themselves to anybody without prejudice, which seems like a lovely trait, but at the end of the day there is a reason why the human brain evolved to be a little bit suspicious of a new person. You don't know if that person is there to hurt you or to love you, and they [a person with WS] cannot make that distinction," he says.

Few people with WS live independently as adults, and many suffer from severe anxiety. There are also health problems that accompany the condition, such as cardiovascular disease, developmental delays, and learning disabilities. Many people with WS have a lower IQ than the average, for example.

Over the last decade, scientists have learned more about the condition, which is offering a unique window into how some of the traits that make us human – such as kindness, trust, and friendliness – evolved.

First off, some facts. Humans have 46 chromosomes, organised into 23 pairs. During sperm or egg development, a process called 'recombination' occurs where genetic material is swapped between matching pairs of chromosomes. However in WS, the process goes wrong, and a whole section of DNA from one copy of chromosome seven is accidentally deleted. As a result, people with WS are missing one copy of between 25-27 genes.

These genes serve various functions. For example one, ELN, codes for a protein called elastin, which provides flexibility and elasticity to tissues throughout the body. A lack of elastin causes the artery walls to stiffen, leading to lifelong cardiovascular problems for people with Williams Syndrome.

Another gene, BAZ1B, affects the growth of what are known as neural-crest cells. These are stem cells which eventually form the basis of many tissues, including the bones and cartilage of the face. People with Williams Syndrome have distinct facial characteristics, such as a small, upturned nose, wide mouth, and small chin.

Yet pinpointing the gene, or genes, responsible for the increased friendliness of people with WS has proved more elusive. One theory is that

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