Frank Drake is famous for his Drake equation which attempts to estimate the amount of intelligent life in the universe.  It is actually fairly simple.

N = R* × f_p × n_e × f_l × f_i × f_c × L

where:

• N is the number of civilizations in our galaxy with which we might hope to be able to communicate;

and

• R^* is the average rate of star formation in our galaxy
• f_p is the fraction of those stars that have planets
• n_e is the average number of planets that can potentially support life per star that has planets
• f_ℓ is the fraction of the above that actually go on to develop life at some point
• f_i is the fraction of the above that actually go on to develop intelligent life
• f_c is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
• L is the length of time such civilizations release detectable signals into space.

Stephen Hawking

Stephen Hawking comes along in this speech and essentially, from what I can gather, argues that the first four variables/functions hold potentially high values. 

The early appearance of life on Earth suggests that there's a good chance of the spontaneous generation of life, in suitable conditions.

He then lays out four reasons as to why we haven't been contacted yet.

He subscribes mostly to point 2.  Wherein the probability of life is reasonable but the probability of intelligence is low.  Which implies that the last three of Drakes functions/variables are quite low.

Seems somewhat probable due to the current lack of evidence to the contrary. So if you treat the Drake equation as merely a framework and plug in realistic numbers that reflect the likely rarity of intelligent life you get a very low chance of encountering an intelligent species out there.