A low IQ generally refers to a score below 85 on a standardized intelligence test, though the term covers a wide range of functioning. The average IQ is 100, with most people scoring between 85 and 115. Scores below that threshold fall into progressively lower categories, each with different real-world implications.
How IQ Scores Are Classified
IQ tests like the Wechsler and Stanford-Binet scales use a scoring system built around an average of 100 and a standard deviation of 15 points. That means most people cluster near the middle, and scores spread out symmetrically in both directions. The standard classification breaks down like this:
- Low average: 80 to 89
- Borderline: 70 to 79
- Extremely low: below 70
About 16% of the population scores below 85, and roughly 2.5% scores below 70. These categories aren’t just academic labels. They correspond to meaningful differences in how easily a person picks up new skills, handles abstract thinking, and manages daily tasks independently.
Borderline Intellectual Functioning
People who score between 70 and 85 fall into a category sometimes called borderline intellectual functioning, or BIF. This isn’t a formal diagnosis on its own. In the DSM-5-TR (the main diagnostic manual used by mental health professionals in the U.S.), it’s listed as a “v-code,” meaning it’s a condition that can be noted and addressed but doesn’t qualify as a standalone disorder.
People with BIF often experience some of the same challenges as those with intellectual disability, just to a lesser degree. They may struggle with reading comprehension, managing money, or navigating complex social situations. These difficulties can affect school performance, job prospects, and independence, but many people in this range live fully independent lives with the right support and accommodations.
When Low IQ Becomes a Diagnosis
An IQ of approximately 70 or below, roughly two standard deviations below the average, is the threshold associated with intellectual disability. But a low score alone isn’t enough for a diagnosis. The DSM-5 specifically moved away from relying on IQ numbers as the defining factor. Instead, clinicians assess how well a person functions across three domains of everyday life.
The first is conceptual: skills like language, literacy, understanding time, and working with numbers. The second is social: interpersonal skills, social responsibility, the ability to follow rules, and awareness of when someone might be taking advantage of you. The third is practical: personal care, job skills, using transportation, managing schedules, handling money, and staying safe.
A person with an IQ of 68 who holds a job, maintains relationships, and handles daily routines may not meet the criteria for intellectual disability. Someone with an IQ of 72 who struggles significantly in those areas might. The emphasis is on real-world functioning, not a number on a test.
What Causes a Low IQ
Low IQ scores can result from genetic factors, environmental exposures, or complications during pregnancy and birth. In many cases, especially with milder deficits, the cause involves a combination of inherited traits and life circumstances, much like height or weight.
For more significant intellectual disability, specific causes are more commonly identified. These include de novo genetic mutations (new mutations not inherited from either parent), genetic imprinting conditions, and chromosomal differences like Down syndrome. On the environmental side, oxygen deprivation around birth, maternal infections during pregnancy such as rubella, exposure to lead or other toxins, preterm birth, and gestational diabetes have all been linked to lower cognitive outcomes.
Nutrition plays a role too, particularly during early childhood. Severe malnutrition, iodine deficiency, and iron deficiency during critical developmental windows can all affect brain development in ways that show up on IQ testing years later.
How Low IQ Affects Daily Life
The practical impact of a low IQ depends enormously on where someone falls on the spectrum and what support systems are in place. A person scoring in the low average range (80 to 89) may find school more challenging than their peers, need more time to learn new job tasks, or prefer concrete instructions over abstract ones. Most function independently without any formal support.
In the borderline range (70 to 79), challenges become more visible. Academic work that relies on reading comprehension, abstract reasoning, or multi-step problem solving tends to be difficult. Socially, people in this range may be more trusting than is safe, making them vulnerable to manipulation or exploitation. Managing finances, understanding contracts, and navigating bureaucratic systems like insurance or taxes can be overwhelming without guidance.
Below 70, the impact is more pervasive. People in this range typically benefit from structured support in education and employment. Supported employment services, where a job coach works alongside the person, help many individuals succeed in competitive jobs they couldn’t manage alone. Person-centered planning, an approach that puts the individual’s own goals and preferences at the center of decisions, has become the standard for designing that support.
IQ Scores Are Not Fixed
One important thing to understand is that IQ scores can shift over a person’s lifetime, and they’ve shifted across entire populations over time. For most of the 20th century, average IQ scores rose steadily from one generation to the next, a phenomenon known as the Flynn effect. This rise, roughly 3 points per decade in many countries, was driven by improvements in nutrition, education, and environmental conditions.
That trend has recently reversed in many wealthy nations. Studies of test scores from 2000 to 2018 show that the most economically advanced countries are now seeing flat or slightly declining scores, while developing countries continue to gain. The gap between the highest and lowest scoring regions is shrinking, with differences as large as 19 IQ points between regions like East Asia and South Asia, but these gaps are narrowing over time.
This matters because “low IQ” is always defined relative to the current population average. The cutoffs of 70 and 85 aren’t measuring some absolute capacity. They reflect where a person stands compared to everyone else taking the same test at the same time. A score that qualifies as borderline today might have been classified differently a generation ago.
What IQ Tests Don’t Measure
IQ tests capture a specific set of cognitive skills: pattern recognition, working memory, processing speed, verbal reasoning, and spatial thinking. They do not measure creativity, emotional intelligence, motivation, practical wisdom, or the kind of street smarts that help people navigate real-world problems. A person with a low IQ score may excel in areas that standardized tests simply don’t assess.
Cultural and language barriers can also depress scores. Someone taking a test in their second language, or one designed around cultural references they didn’t grow up with, may score lower than their actual cognitive ability warrants. This is one reason clinicians are trained to interpret IQ scores in context rather than treating them as definitive measures of a person’s potential.