Hi Lorraine,

Thank you for contacting the UNH Extension Yard and Garden Infoline with your concern about the leaves on your dumb cane (Dieffenbachia) turning yellow.  As you suggested, one of the major causes of yellowing leaves is overwatering.  Leaf yellowing can also be caused by your plant being in full sunlight.  

I have included a link to a fact sheet that discusses more fully how best to grow a dumb cane.

https://homegarden.cahnr.uconn.edu/factsheets/dieffenbachia/

The following fact sheet suggests that the yellowing of lower leaves is natural.  It states,  "When the lower leaves turn yellow, remove them. If the stem becomes bare and objectionable the top can be cut off and rooted in sand or water, the air layering method works, too."  From the information in your email, I am unsure if only the lower leaves on your plant are turning yellow. 

https://extension.illinois.edu/houseplants/dieffenbachia

https://extension.psu.edu/dieffenbachia-diseases  This fact sheet notes, when lower leaves yellow, wilt and die,  the cause can be lack of water and an even soil moisture should be maintained.  

The following fact sheet also mentions that the yellowing of lower leaves is a part of the plant's natural growth pattern.

https://gardeningsolutions.ifas.ufl.edu/plants/houseplants/dieffenbachia/#:~:text=Leaf%20Problems&text=Move%20your%20plant%20to%20an,of%20your%20regular%20up%2Dkeep.

 I hope the information provided will help you with the care of your plant.