For most of us, the idea of not being able to attend school because of one’s gender is unthinkable. But in developing countries, particularly in Africa, it’s all too common for girls to be excluded from formal education. In Sub Saharan Africa, some 16 million girls are not in school. How fitting that this year’s celebration of International Day of the Girl focuses on education.

Why aren’t these girls in school? It’s a tough question to answer— complex social, political, traditional, and economical factors all play a part. Everything from marriage practices to disease, poverty, and seasonal labor requirements contribute to this catastrophe. The result is devastating—only one in four poor African girls attends school.