You're right. It's been proven a bunch of times that people are more likely to leave negative reviews than positive ones, or to remember bad experiences more overall.

Zendesk actually did a survey on this back in 2013:

http://cdn.zendesk.com/resources/whitepapers/Zendesk_WP_Cust...

And American Express found something similar in their Global Customer Service Barometer survey as well:

http://about.americanexpress.com/news/docs/2014x/2014-Global...