Learning to read by the end of third grade is crucial, those who study early childhood education say, because that’s the point at which children start using reading to learn other subjects. Those who are proficient in reading by the end of third grade are much more likely to graduate from high school, and to be economically successful as adults.

But about two-thirds of students in fourth grade don’t meet reading proficiency standards, according to the National Assessment of Educational Progress and the National Center for Education Statistics. And those numbers are much higher for students who come from low-income households.

Four of every five students who come from low-income households don’t meet reading proficiency requirements, a new report from the Annie E. Casey Foundation finds. The numbers are worst among minority communities: 83 percent of African-American fourth graders don’t meet proficiency standards, while 81 percent of Hispanic and Latino children fall short. More than nine in 10 dual-language learners are below proficiency by fourth grade, the report finds.

Percentage of low-income students who fall below reading proficiency

(Source: Annie E. Casey Foundation)

That’s not to say students who come from wealthier homes are doing particularly well: More than half of all high-income students fall short of reading proficiency levels in 27 states, the Casey Foundation reported.

Percentage of high-income students who fall below reading proficiency

(Source: Annie E. Casey Foundation)

Eleven states and the District of Columbia have seen scores increase by at least 10 percentage points between 2003 and 2013. The biggest drop in non-proficiency came in Maryland, where just 32 percent of students were proficient in 2003. A decade later, that number jumped to 45 percent.

But low-income students accounted for far less of that improvement than their high-income peers. Higher-income students showed a 17 percent increase in proficiency levels, while low-income students improved by just 6 percentage points. The gap between low-income students and higher-income students increased in nearly every state over the last decade.

And that gap matters. It “often starts early as a result of health problems at birth, contributing to lags in language and social-emotional skill development in early childhood,” the report said. “Low-income children are also more likely to miss out on high-quality early learning experiences, which can help mitigate these delays.”

Higher-income students meet reading proficiency levels most often in Massachusetts, where 62 percent hit proficiency targets, and the District of Columbia, where 61 percent of students measure up.

Low-income students do worst in Arizona, Alaska, California, Louisiana, Mississippi and New Mexico, the survey found. In all six states, 85 percent of students from low-income households fail to meet proficiency standards.