I use forty years as the measure as that seems to stretch from the age of the hippies (i.e a crucial touchstone in marijuana's cultural history) to modern day. Now this is a pretty arbitrary measure, but it serves to illustrate the change in marijuana, and also allows for people to document the change (or at least their impression of it). In short, if you ask people who used to smoke at that time, they would say that marijuana is stronger now than it was back then.
Evidence would suggest that this is an empirical change in product and not just an affect of time and retrospect. Reports show that pot in the 1970s had THC levels of around 1%. However, today, the herb you’re smoking has a lot more THC, with levels averaging between 6-8% . Some specially grown plants can contain THC levels as high as 51%.
There are numerous reasons for this change. If you're growing your own then the internet, for example, provides you with the information needed to improve your grow. Similarly, developments in technology have enabled the increase in strength in a myriad of ways at all stages of the production line. The spikes in THC levels today are also caused by better growing climate conditions and how they harvest and process cannabis.
In the past, the cannabis that was available was a mix of leaves, stems and flowers. So hippies were consuming parts of the plant that were not rich in THC. Dispensaries today sell product that contains more of the feminized flowers (sinsemilla), which contains higher THC levels and a better quality product.