Do gears cause laziness?

I celebrated the start of 'Summer' by switching from my singlespeed to my geared Peregrine and noticed that my average speed dropped significantly. The Peregrine is a little lot heavier but I don't spin out at about 24 mph so the 2 should balance each other out. Looking at my riding I realised that whereas on the singlespeed I just push harder when the road got steeper or the wind blew on the geared bike I'd just drop a gear or 3 and spin. So today I deliberately pushed myself, staying in a higher gear and managed to achieve a higher average speed (I also achieved a new top speed of 80 mph but suspect that there might be a degree of measurement error!!).