Quickly looking at the research, seems you are right. Like most diseases, it's interplay between genetics and environment.
Social support is less important risk factor, while factors like marital satisfaction and partner's depression are more important. For women, adverse events such as loss of a loved one, husband's loss of job or serious economical issues seem to be important too (Escriba-Aguir et al. 2010).
This is also quite concordant with the fact that PPD is more common in less developed countries, as mentioned in another comment.
Social support is less important risk factor, while factors like marital satisfaction and partner's depression are more important. For women, adverse events such as loss of a loved one, husband's loss of job or serious economical issues seem to be important too (Escriba-Aguir et al. 2010).
This is also quite concordant with the fact that PPD is more common in less developed countries, as mentioned in another comment.