Title: Weight Expansion: A New Perspective on Dropout and Generalization. Abstract: While dropout is known to be a successful regularization technique, insights into the mechanisms that lead to this success are still lacking. We introduce the concept of “weight expansion”, an increase in the signed volume of a parallelotope spanned by the column or row vectors of the weight covariance matrix, and show that weight expansion is an effective means of increasing the generalization in a PAC-Bayesian setting. We provide a theoretical argument that dropout leads to weight expansion and extensive experimental support for the correlation between dropout and weight expansion. To support our hypothesis that weight expansion should be regarded as the cause for the increased generalization capacity obtained by using dropout, and not just as a mere by-product, we have studied other methods that achieve weight expansion (resp. contraction), and found that they generally lead to an increased (resp. decreased) generalization ability. This suggests that dropout is an attractive regularizer because it is a computationally cheap method for obtaining weight expansion. This insight justifies the role of dropout as a regularizer, while paving the way for identifying regularizers that promise improved generalization through weight expansion.