stress is usually a cause of hermaphroditing, and I've heard genetics can be a factor as well. example, I helped a friend that I'm a caregiver for move a couple plants across town because our apartments were being inspected. so anyway, before we moved them they were starting to flower beautifully. a few days after we brought the plants back, we noticed pollen sacks developing on several of the plants. so yea, extreme fluctuations in temperate and humidity, as well as disruptions to the plants current light cycle can be very stressful on plants.
I would broadly agree with stonedcold. However, I am convinced that genetics is THE main cause of 'hermies'. Certainly stress is a factor, but I also think that growers now tend to overplay the 'stress' factor and often you will read of plants being described in almost human terms! In 12 years of 'serious' growing I have seen only a few of them, but almost always from seeds/strains that have been 'feminised' or repeatedly cross bred. I have grown f2's for example that have NEVER produced a hermie, grow after grow, and I can confidently say that its not going to happen unless there is something like
stonedcold has desrcribed above, ie sudden dramatic swing in temps, humitidy etc....ie: a 'shock'.
I don't really buy into this theory about the dark period having to be 100% 'dark' or it will cause a plant to hermie either! I have grown plants outdoors in the garden right outside the kitchen window and with patio light going on and off etc......... NEVER had an outdoor plant hermie. What I would say is that plants acclimatise early on to the environment and to the level of light that is 'normal' during the 'night' period. Think about a full moon for example...... its incredibly 'light' but who has ever known an outdoor plant to hermie? I have not.
Occasionally one will see the odd late 'banana' in flowering that will result in a couple, literally just one or two seeds, but I regard this as totally normal and natural as the female plants nature is to reproduce, but this is very different from a full blown hermie that will show both male and female flowers at an early stage. I have grown out such seeds and they have flowered perfectly normally.
I will be interested to read others experiences and opinions in reply to this thread