Our Gemara on Amud Beis records a statement made in frustration by Rav Adda bar Ahava that ended up as a curse against the children of Rav Huna. The words of a sage are so powerful, that even when not fully intended, they have an effect.

This idea that words affect reality is a highly regarded principle mentioned dozens of times in the Gemara. Sanhedrin (102a) characterizes it as a covenant between one’s lips and God.  Sefer HaChinuch (231) says this principle is based on the spiritual power of every person to affect reality with their wishes. This is not only for curses, but blessings as well. Thus Rabbi Yochanan (Moed Kattan 18a) tells us that Avraham absentmindedly mentioned that he would return to the encampment with Isaac, even though he was planning on  sacrificing him as per God’s command. In the end, Isaac DID return with him, and Moed Kattan attributes this outcome to his words.  This power applies to Gentiles as well, as per the Gemara in Sotah (42b) that Goliath predicted his own demise and even caused it by scoffing incredulously that David might smite him. Another example comes from Gemara (Bava Metzi’a 68a) where the mere mentioning of a great sage’s possible demise led to his actual demise. There is a similar idea known as, “al tiftach peh le-Satan”, which literally means, “do not give Satan (the accuser) an opening argument”, i.e., do not speak evil of yourself, as this will allow Satan to contend that you deserve some kind of suffering (see Kesuvos 8b),

So far we have discussed the spiritual aspects of this principle.  However it is also an accepted psychological principle that beliefs and scripts in one’s head lead to self-fulfilling prophecies.  As in regard to this spiritual principle, the effect seems to be for both positive and negative. The effect is due to multiple co-reinforcing factors such as positive self-perception and expectations leads to more confidence, and to seeking more opportunities for social, emotional and material competencies. Researchers Sonja Lyubomirsky And Ed Diener (“The Benefits of Frequent Positive Affect: Does Happiness Lead to Success?”, Psychological Bulletin Copyright 2005 by the American Psychological Association 2005, Vol. 131, No. 6, 803–85.) state:

“…happy people—those who experience a preponderance of positive emotions—tend to be successful and accomplished across multiple life domains. Why is happiness linked to successful outcomes? We propose that this is not merely because success leads to happiness, but because positive affect (PA) engenders success. Positively valenced moods and emotions lead people to think, feel, and act in ways that promote both resource building and involvement with approach goals (Elliot & Thrash, 2002; Lyubomirsky, 2001). An individual experiencing a positive mood or emotion is encountering circumstances that he or she interprets as desirable. 

Positive emotions signify that life is going well, the person’s goals are being met, and resources are adequate (e.g., Cantor et al., 1991; Carver & Scheier, 1998; Clore, Wyer, Dienes, Gasper, & Isbell, 2001). In these circumstances, as Fredrickson (1998, 2001) has so lucidly described, people are ideally situated to “broaden and build.” In other words, because all is going well, individuals can expand their resources and friendships; they can take the opportunity to build their repertoire of skills for future use; or they can rest and relax to rebuild their energy after expending high levels of effort. Fredrickson’s model (Fredrickson, 2001) suggests that a critical adaptive purpose of positive emotions is to help prepare the organism for future challenges. Following Fredrickson, we suggest that people experiencing positive emotions take advantage of their time in this state—free from immediate danger and unmarked by recent loss—to seek new goals that they have not yet attained (see Carver, 2003, for a related review).

The characteristics related to positive affect include confidence, optimism, and self-efficacy; likability and positive construals of others; sociability, activity, and energy; prosocial behavior; immunity and physical well-being; effective coping with challenge and stress; and originality and flexibility. What these attributes share is that they all encourage active involvement with goal pursuits and with the environment. When all is going well, a person is not well served by withdrawing into a self-protective stance in which the primary aim is to protect his or her existing resources and to avoid harm—a process marking the experience of negative emotions. Positive emotions produce the tendency to approach rather than to avoid and to prepare the individual to seek out and undertake new goals. Thus, we propose that the success of happy people rests on two main factors. First, because happy people experience frequent positive moods, they have a greater likelihood of working actively toward new goals while experiencing those moods. Second, happy people are in possession of past skills and resources, which they have built over time during previous pleasant moods.

However, there is one caveat: According to researchers David de Meza and Chris Dawson (“Neither an Optimist Nor a Pessimist Be: Mistaken Expectations Lower Well-Being”, Personality and Social Psychology Bulletin 1–11, The Society for Personality and Social Psychology, Inc.):

“…Mental health requires accurate self-perception. Using a representative British sample (N = 1,601) it finds that, as measured by two established well-being indicators, those with mistaken expectations, whether optimistic or pessimistic, do worse than realists. …Plans based on inaccurate beliefs are bound to deliver worse outcomes than would rational expectations.”

Blithe and superficial optimism can be damaging as it promotes an inflated, grandiose and non-realistic understanding of a person’s interaction and relation to the world.  A person who professes positivity and optimism without having an ability to face and assess deficits and mistakes, will be doomed to live in a false bubble which will pop rudely from time to time when various bottoms are encountered. Yet, appropriate optimism and positive belief orients a person to make the cognitive and social connections to take the necessary risks for success.