|If company newsletters truly reduced turnover, that would be news. Granted these newsletters can be well-received and a source of interesting information about the company and the employees, it is a huge leap to directly link them to reductions in turnover.|
Since your real interest is focused on reducing turnover, the most important point to note is that turnover is primarily caused by dissatisfaction. When the employees' needs are not being adequately met on one job, they are likely to look elsewhere to have them satisfied.
You should consider using a combination of employee surveys, open discussions with employees at all levels, managing by wandering around, and exit interviews to gather a more thorough picture of the reasons why people are leaving. The causes can include any factor or factors associated with working at your company, such as the pay, benefits, working conditions, supervision, recognition, communication, and advancement opportunities, to name just a few.
By paying very careful attention to reasons behind the turnover, you should be able to implement policies and programs to stem the tide. Since newsletters can be used to publicize these policies and programs, they may be correlated with reduced turnover, but they do not cause it.