Most people are under the impression that wood is a static material like metal or stone.
Conversely, in the same atmospheric conditions, solid wood movement in length is negligible. The best way to visualize this is to imagine a tree as millions of tiny soda straws filled with water. All these straws are standing on end and are closely bundled. When the tree is cut into lumber and dried, the cells or straws lose their moisture and collapse. Being long cell structure most of the shrinkage is in the width of the board.