A recent paper in Cortex describes how caloric vestibular stimulation (CVS), i.e., rinsing of the ear canal with cold water, reduces unrealistic optimism. Here are some bits from the paper:

Participants were 31 healthy right-handed adults (15 men, 20–40 years)...

Participants were oriented in a supine position with the head inclined 30° from the horizontal and cold water (24 °C) was irrigated into the external auditory canal on one side (Fitzgerald and Hallpike, 1942). After both vestibular-evoked eye movements and vertigo had stopped, the procedure was repeated on the other side...

Participants were asked to estimate their own risk, relative to that of their peers (same age, sex and education), of contracting a series of illnesses. The risk rating scale ranged from −6 (lower risk) to +6 (higher risk). ... Each participant was tested in three conditions, with 5 min rest between each: baseline with no CI (always first), left-ear CI and right-ear CI (order counterbalanced). In the latter conditions risk-estimation was initiated after 30 sec of CI, when nystagmic response had built up. Ten illnesses were rated in each condition and the average risk estimate per condition (mean of 10 ratings) was calculated for each participant. The 30 illnesses used in this study (see Table 1) were selected from a larger pool of illnesses pre-rated by a separate group of 30 healthy participants.Overall, our participants were unrealistically optimistic about their chances of contracting illnesses at baseline ... and during right-ear CI. ...Post-hoc tests using the Bonferroni correction revealed that, compared to baseline, average risk estimates were significantly higher during left-ear CI (p = .016), whereas they remained unchanged during right-ear CI (p = .476). Unrealistic optimism was thus reduced selectively during left-ear stimulation.

(CI stands for caloric irrigation which is how CVS was performed.)

It is not clear how close the participants came to being realistic in their estimates after CVS, but they definitely became more pessimistic, which is the right direction to go in the context of numerous biases such as the planning fallacy.

The paper:

Vestibular stimulation attenuates unrealistic optimism

• Ryan McKay, 
• Corinne Tamagni, 
• Antonella Palla, 
• Peter Krummenacher, 
• Stefan C.A. Hegemann, 
• Dominik Straumann, 
• Peter Brugger