Imbalances in phylogenetic diversity could be the result of variable unbounded diversification rates, differing limits on diversity, or a combination of the two. We propose an approach to distinguish between rates and limits as the primary cause of phylogenetic imbalance, using parasitic plants as a model. With sister-taxon comparisons, we show that parasitic plant lineages are typically much less diverse than their autotrophic sisters. We then use age estimates for taxa used in the sister-taxon comparisons to test for correlations between clade age and diversity. We find that parasitic plant diversity is not significantly correlated with the age of the lineage, whereas there is a strong positive correlation between the age and diversity of autotrophic sister lineages. The Ericaceae sister-pair Monotropoideae (parasitic) and Arbutoideae (autotrophic) are sufficiently well samples at the species level to allow more parametric comparisons of diversification patterns. Model-fitting for this group supports ecological limitation in Monotropoideae and unconstrained diversification in Arbutoideae. Thus, differences in diversity between parasitic plants and their autotrophic sisters might be caused by a mixture of ecological limitation and unbounded diversification. A combination of sister-taxon comparisons of diversity and age, coupled with model-fitting of well-sampled phylogenies of focal taxa, provides a powerful test of likely causes of asymmetry in the diversity of lineages.