Some people have known vulnerabilities to depression. Yet from our shared nature as human beings, depression can affect anyone. Depression has been observed in all cultures and all known periods of history. Depression-like behaviour has also been observed across the animal kingdom, even in species as far down the evolutionary tree as crustaceans. The pattern is so common and widespread that some researchers see depression not as a disease, like cancer, but as an innate response, like fainting. We all have the inbuilt machinery that produces depression, but for some people, their trigger-system is more sensitive than others. The sensitivity could be a collection of genes, which is why family history is a risk factor. The sensitivity could also come from adverse experiences in early life, such as the death of a parent. Later in life, other elements may arrive to weaken defences, such as alcohol, drugs, or medical illnesses. Then come the manifold stresses of life, any of which can form the tipping point.