Now Steven D. Levitt and John List, of the NY Times Freakonomics blog say they have looked at the original data and find it does not support the conclusion that is primarily described as the 'Hawthorne effect'. A more complete write-up in the Economist.
' ... It turns out that idiosyncrasies in the way the experiments were conducted may have led to misleading interpretations of what happened. For example, lighting was always changed on a Sunday, when the plant was closed. When it reopened on Monday, output duly rose compared with Saturday, the last working day before the change, and continued to rise for the next couple of days. But a comparison with data for weeks when there was no experimentation showed that output always went up on Mondays. Workers tended to beaver away for the first few days of the working week in any case, before hitting a plateau and then slackening off ... 'Points to classic issues of data collections and external influences. You still have to consider the effect of observation on real people.