The two most widely recognized causes of Parkinson’s disease are genetics and environmental exposures. Most researchers now believe that the majority of cases involve both factors working together: inherited vulnerabilities in certain genes make brain cells more susceptible to damage from environmental triggers like pesticides, head injuries, or other toxins. Only a small fraction of cases trace back to a single clear-cut cause.
Genetics: The Internal Risk Factor
About 15 to 25 percent of people with Parkinson’s have a family history of the disorder, and scientists have identified mutations in at least 15 genes that can directly cause the disease. The first genetic link was discovered in 1997 with mutations in the SNCA gene, which provides instructions for making a protein called alpha-synuclein. When this protein misfolds and clumps together inside brain cells, it disrupts normal cell functions, damages the energy-producing structures within those cells, and interferes with the connections between neurons.
The most common single-gene cause identified so far involves mutations in a gene called LRRK2, discovered in 2004. The most frequent LRRK2 variant accounts for roughly 1 percent of all Parkinson’s cases with no clear family history and about 4 percent of cases where Parkinson’s runs in the family. Other gene mutations, including PRKN, PINK1, and DJ-1, tend to cause earlier-onset forms of the disease, sometimes appearing in people under 50.
Still, all known single-gene causes combined explain only about 30 percent of cases with a family history and just 3 to 5 percent of cases that appear to arise randomly. That gap points strongly to a second category of cause.
Environmental Exposures: The External Trigger
Pesticides are among the best-studied environmental risk factors. Two chemicals stand out in the research: rotenone (an insecticide once widely used in farming and fishing) and paraquat (an herbicide still used in many countries). A major human exposure study found that people with Parkinson’s were 2.5 times more likely than healthy controls to have used rotenone or paraquat. Rotenone damages the energy-producing machinery inside brain cells, while paraquat generates a type of molecular stress that destroys the same dopamine-producing neurons that Parkinson’s targets.
Head trauma is another significant environmental contributor. A meta-analysis combining 15 studies found that people with a history of traumatic brain injury had a 48 percent higher risk of developing Parkinson’s later in life. This is one reason the disease has drawn attention in contact sports and among military veterans.
There’s also a compelling theory that the disease process may begin outside the brain entirely. Research suggests that the damaging protein clumps characteristic of Parkinson’s can start forming in the gut’s own nervous system or the olfactory bulb (which processes smell) and then spread along nerve pathways into the brain. This raises the possibility that ingested or inhaled environmental substances could kick off the disease long before motor symptoms appear, which might explain why digestive issues and loss of smell often precede tremors and stiffness by years.
How Both Causes Work Together
The National Institute of Neurological Disorders and Stroke notes that in most cases, the cause of Parkinson’s remains officially unknown. But the prevailing scientific view is that most, if not all, cases probably involve both a genetic and an environmental component. A person might carry gene variants that make their brain cells slightly less efficient at clearing damaged proteins or handling oxidative stress. On their own, those variants might never cause problems. Add decades of low-level pesticide exposure, a head injury, or other environmental insults, and the cumulative burden crosses a threshold.
At the cellular level, both genetic and environmental causes converge on the same damage pathway. The energy-producing structures inside dopamine neurons (mitochondria) become impaired, toxic protein clumps accumulate, and the neurons in a brain region called the substantia nigra gradually die off. By the time visible symptoms like tremor, stiffness, and slowness of movement appear, a substantial portion of those neurons are already gone.
Factors That Lower Risk
Interestingly, some common substances appear to offer a degree of protection. Pooled data from multiple studies show that current smokers have roughly a 60 percent lower risk of Parkinson’s compared to people who have never smoked, and coffee drinkers show about a 30 percent reduction in risk compared to non-drinkers. Nicotine and caffeine both reduce dopamine neuron damage in animal models of the disease. Caffeine appears to work by blocking specific receptors concentrated in dopamine-rich areas of the brain. These findings don’t mean smoking is a health strategy, but they’ve pointed researchers toward potential protective mechanisms that could inform treatments.
A Growing Global Challenge
Parkinson’s is the fastest-growing neurological disorder in the world. A 2024 modeling study published in The BMJ projected that 25.2 million people will be living with Parkinson’s globally by 2050, a 112 percent increase from 2021. Men are affected at higher rates than women, with an estimated 295 cases per 100,000 men compared to 243 per 100,000 women by mid-century. Population aging accounts for much of this growth, but it also underscores why understanding both genetic and environmental causes matters: prevention strategies that reduce even one category of risk could slow an enormous wave of disease.