I have some datapoints here. Twice in the last five years I have spent a year working half time (German parental leave is incredible). I noticed spending a higher proportion of my time on 'overhead' - emails, code reviews etc, but my overall productivity did not drop.
I measured the bugs fixed over time, commits made, lines and files changed, emails sent, all proxies for activity.
Bear in mind when looking at the charts that they are corrected for man months not calendar months - my calendar productivity remained about the same or slightly better over the measured period.
I recently reduced my hours to 27.5 just by asking. Without these positive experiences for all concerned that may not have been so easy.
I measured the bugs fixed over time, commits made, lines and files changed, emails sent, all proxies for activity.
Some results are shown in this otherwise tangentially related blog post: http://yieldthought.com/post/6070927890/metagame-productivit...
Bear in mind when looking at the charts that they are corrected for man months not calendar months - my calendar productivity remained about the same or slightly better over the measured period.
I recently reduced my hours to 27.5 just by asking. Without these positive experiences for all concerned that may not have been so easy.