Definition
The Pareto principle is a rule of thumb based on observation: roughly 80% of results come from just 20% of causes. In many situations, causes and effects don't split fairly 50/50. A small group of key causes produces most of the results.
The Closet and the Ant Colony
Open your closet every morning and it's packed with clothes. Yet you keep reaching for the same few favorites. The clothes you wear most often tend to be a small part of what you own.
Uneven effort shows up in nature, too. In a long-term study of ant colonies in Japan, 20 to 30% of the worker ants did almost no work. When the busy ants disappeared, some of the resting ants took over. But the point of that study is that the idle ants act as a reserve workforce, so it isn't the same thing as the Pareto principle.
Still, a lot of what we see around us works this way. Effort and results rarely match up one-to-one. A few key factors often decide most of the outcome, or even the overall mood.
The Pea Pod Story and Italian Land
The principle is named after Vilfredo Pareto, an Italian economist. A popular story says he noticed it in his garden, when 80% of his peas came from just 20% of the pods. But there's no record to back that story up.
What we do know is that in his writings from the 1890s, he described how wealth was concentrated. For example, about 80% of the land in Italy was owned by 20% of the people. The person who gave this observation the name "Pareto principle" and stretched it into a general idea was Joseph Juran, a quality-control expert, in the 1940s.
After that, business thinkers started applying it to the workplace. People often found lopsided patterns like these: 80% of a department store's sales come from the top 20% of its regular customers, and 80% of software bugs come from 20% of the code. That helped the idea spread widely.
A More Accurate Way to Put It
You don't need to get hung up on the numbers 80 and 20. The 20% (causes) and the 80% (results) measure two different things, so they don't have to add up to 100. Depending on the situation, the top 20% might produce 70% of the results, or the top 10% might produce 80%.
The real lesson of the Pareto principle is "focus on what matters." If you try to put equal effort into everything, you'll burn out fast. Think about studying for an exam. Instead of memorizing every line in the textbook with equal weight, you might lock down the key concepts that come up most often first.
The heart of this principle is the wisdom to figure out where your energy will pay off the most.
Common misconceptions
Since 80 and 20 add up to 100, it's an absolute math formula where the total must always be 100%.
Causes and results are measured on different scales. Just as the top 10% of causes can produce 80% of results, or 20% of causes can produce 70%, the two numbers don't have to add up to 100. The key idea is the imbalance between causes and results.
Where you meet it
A large share of a department store's sales comes from a small group of top customers (VIPs).
Even with dozens of apps on your smartphone, you often use only a handful regularly.
At work, most of your important results can come from a small portion of the hours when you were really focused.
In one sentence
The Pareto principle is the observation that most results (80%) often come from a few key causes (20%), which leads to the idea of focusing on what matters most.