There are likely to be a complex array of interconnecting factors underpinning this, and it is not possible to draw conclusions on the basis of the current evidence base.
Mae'n debygol y bydd amrywiaeth gymhleth o ffactorau rhyng-gysylltiedig yn sail i hyn, ac nid yw'n bosibl dod i gasgliadau ar sail y sylfaen dystiolaeth gyfredol.