Several studies have established specific relationships between White Matter (WM) and behaviour. However, these studies have typically focussed on fractional anisotropy (FA), a neuroimaging metric that is sensitive to multiple tissue properties, making it difficult to identify what biological aspects of WM may drive such relationships. Here, we carry out a pre-registered assessment of WM-behaviour relationships across multiple behavioural, anatomical and biological domains. Surprisingly, we find support for predicted relationships between FA and behaviour only in one of three pre-registered tests. We also find no evidence for consistent multimodal signatures across neuroimaging markers with different biological sensitivity, which suggests there is no common biological substrate for WM-behaviour relationships. These results demonstrate that FA-behaviour relationships from the literature may not be easily generalisable across domains. They also highlight a broad heterogeneity in WM's relationship with behaviour, indicating that variable biological effects may be shaping their interaction.