American students aren't particularly good at math, and a new study confirms that fact.

The average math score for American 15-year-olds dropped to the lowest point since 2006, according to a 2015 international assessment of education systems around the world, released on Tuesday

Which raises the question again: Why are Americans so bad at math?

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A 2014 story in The New York Times Magazine by education expert Elizabeth Green attempted to answer that question.

Green compared math instruction methods in America and Japan, which consistently ranks in the top five countries in terms of math literacy.

Green concluded that the U.S. emphasizes memorization and repetitive practice in mathematics classes, which fails to drive home lessons among students.

There's a greater focus in Japan on conceptual learning.

Teachers there develop lesson plans that enable students "to uncover math’s procedures, properties, and proofs for themselves," Green wrote.

Green offered one example from a Japanese elementary school teacher:

"One day, for example, the young students would derive the formula for finding the area of a rectangle; the next, they would use what they learned to do the same for parallelograms. Taught this new way, math itself seemed transformed. It was not dull misery but challenging, stimulating and even fun."

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The difference in teaching styles is in part the product of each country's distinct education system.

• American teachers are required to adhere to federally mandated curricula that limit their ability to craft innovative lesson plans.
• Japanese teachers are encouraged to collaborate with each other and continually experiment with their lesson plans.

The main reason behind America's mathematical shortcomings is that students aren't taught to understand the subject on a conceptual level.

That's the conclusion of Daniel Willingham, a psychology professor at the University of Virginia, who spoke with ATTN:

"What I'm suggesting is that problem starts really, really early — but only becomes evident once kids basically reach mathematics, where you really can't get away with not having conceptual knowledge. Through much of elementary and part of middle school, you can sort of poke along with nothing but algorithmic knowledge — where you're recognizing problem types and properly applying algorithms to them — and the big drop-off comes when you hit algebra, and it becomes much [more] apparent that math is really about reasoning and problem-solving."

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That's why international testing results show American students performing generally well at the elementary school level.

The results dip substantially around the time that students take the Program for International Student Assessment test, around the age of 15.

"If you look at the [Trends in International Mathematics and Science Study test] in the fourth grade and the eighth grade, American kids aren't doing that badly," Willingham said. "They're not in that upper echelon where kids from the East Asian countries are, but they're sort of in the next pack. It's only on the PISA, in 10th grade, that they've really drastically fallen in international rankings.

"The reason there's that big drop-off is because that's when conceptual knowledge becomes really important," Willingham added.