Our Gemara on Amud Beis cautions against making negative and pessimistic declarations, as somehow verbalizing them can cause them to come about.  Jewish tradition most definitely believes in the power of words.  A number of sources that were brought to my attention,, courtesy of Rabbi Moshe Elafant’s sugya shiur on this daf are:

• A Teshuva of the Rashbah (Hameyuchas Leramban 286) states that we cannot know sometimes why bad things happen, but it can be caused by negative statements as there is a special power to words. 

• Sefer Chassidim (479:1) says one must be careful what is said to others and one's children, not to call them bad or predict that they will not turn out not well, as these statements can become self-fulfilling prophecies.

We also know intuitively and psychologically that making positive statements can affect attitudes and experience as well as the opposite.  What does the research say?

According to “The Science of Gratitude”, a white paper prepared for the John Templeton Foundation by the Greater Good Science Center at UC Berkeley, May 2018, the following benefits were noted, and I will quote directly from this paper throughout this article:

• A 1995 study found that when participants felt appreciation, an emotion related to gratitude, their heart rate variability, an indicator of good heart health, improved (McCraty, Atkinson, & Tiller, 1995)
• Not surprisingly, studies have found that more grateful people: report better physical health (Krause & Hayward, 2014), (Hill, Allemand, & Roberts, 2013), are moderately more likely to report engaging in healthy activities (Hill et al., 2013), are more willing to seek help for health concerns (Hill et al., 2013), and sleep better and longer (Mills et al., 2015),(M.-Y. Ng & Wong, 2013. Both optimism and gratitude were associated with higher self-reported adherence to medical recommendations, suggesting that gratitude may help patients better recover from heart attacks and other serious heart problems by encouraging them to follow their doctors’ instructions (Millstein et al., 2016)
• In one study, chronic pain patients with higher trait gratitude reported lower depression and anxiety and better sleep (M.Y. Ng & Wong, 2013)
• Another study suggests that gratitude might help prevent chronic illness. This study found an association between stronger feelings of gratitude and lower levels of hemoglobin HbA1c, a biomarker involved in blood sugar control. High levels of HbA1c have been associated with chronic kidney disease, a number of cancers, and diabetes (Krause et al., 2017)
• People with higher dispositional gratitude report having higher levels of several elements of psychological well-being, including more life satisfaction (McCullough et al., 2002)
• Daily feelings of gratitude are also associated with elements of well-being. A daily diary study found positive relationships between daily feelings of gratitude and feelings of both hedonic (related to pleasure) and eudaimonic (related to meaning and self-realization) well-being (Nezlek, Newman, & Thrash, 2017). Additionally, feelings of gratitude during one day were positively associated with hedonic well-being (though not to eudaimonic well-being) the next day.
• In one study, university students who reported more “higher-order gratitude” (which includes thanking others, thanking God, cherishing blessings, appreciating hardships, and cherishing the moment) also reported more life satisfaction and positive affect, even after controlling for their gender, age, religion, personality traits, and dispositional gratitude (Lin, 2014)
• Another study found that more materialistic undergraduate students reported less life satisfaction, which could be explained by their lower levels of gratitude (Tsang et al., 2014), while a follow-up study found that more materialistic people who were also very grateful did not show lower life satisfaction, suggesting that gratitude may be able to buffer the otherwise negative relationship between materialism and life satisfaction (Roberts, Tsang, & Manolis, 2015)
• A handful of studies have focused on the impact of writing and delivering letters of gratitude on happiness and well-being, particularly letters addressed to people from whom one has received a gift of some kind but whom one has never properly thanked. In one study, people who were asked to write a gratitude letter once a week for three weeks were significantly happier, less depressed, and more satisfied with their lives at the end of the intervention (Toepfer, Cichy, & Peters, 2012)

What are the possible explanations for these effects? The paper hypotheses the following factors:

• Gratitude counteracts hedonic adaptation—when people acclimate to positive developments in their lives and thus do not enjoy them as much—by encouraging people to deliberately focus on what is good in their lives rather than take them for granted.
• Another possibility is that gratitude adheres to Barbara Fredrickson’s “broaden and build” theory of positive emotions (Fredrickson, 2004). This theory posits that positive emotions can broaden a person’s array of thoughts and actions—for example, joy encourages people to play, push their limits, and be creative—and also build a person’s psychological and social resources on which they can rely in difficult times.
• “Gratitude appears to broaden people’s modes of thinking as they creatively consider a wide array of actions that might benefit others,” writes Fredrickson. “Although grateful individuals most typically act prosocially simply to express their gratitude, over time the actions inspired by gratitude build and strengthen social bonds and friendships” (Fredrickson, 2004)
• Yet another way that a positive emotion such as gratitude can build one’s coping capacity is through undoing the effects of negative emotions. Gratitude may combat the negative emotional processes that underpin unhappiness, burnout, and many psychological disorders—as research has indicated that “within momentary experience, positive emotions are incompatible with negative emotions” and that positive emotions can lessen the attention one pays to negative information (Garland et al., 2010)
• However, a recent paper provides theoretical and experimental evidence that gratitude may lead to improved well-being in part by inducing negative emotions. This paper suggests that expressing gratitude motivates people to put effort into a range of positive behaviors (e.g., exercising more, helping others, building relationships) that lead to self-improvement, and it does this by inducing positive emotions—elevation, humility, connectedness—as well as negative ones— indebtedness, guilt, and discomfort (Armenta, Fritz, & Lyubomirsky, 2017)
• Another study found evidence that trait gratitude may have positive effects on well-being partially via its ability to increase self-esteem (Lin, 2015). When a person feels grateful they often view themselves as benefiting from another person’s generosity, leading them to feel valued. This increases self-esteem, which in turn leads to higher levels of psychological well-being.

Gratitude and Trauma

According to this white paper, gratitude affects how trauma is experienced and how recovery takes place:

“Studies have found evidence that more grateful people may be more resilient following traumatic events: Vietnam veterans (Kashdan, Uswatte, & Julian, 2006). 

Gratitude may also lead to more positive psychological changes following a traumatic experience, what researchers call “post-traumatic growth.” Breast cancer patients with higher levels of trait gratitude reported more post-traumatic growth following their experience with cancer as well as reduced distress and more positive emotions, than did patients with lower gratitude (Ruini & Vescovelli, 2013). A study of survivors of a campus shooting found that people with high trait gratitude four months following the shooting showed a stronger relationship between post-traumatic stress and post-traumatic growth, suggesting that very grateful people may be better able to convert stressful situations into opportunities for growth (Vieselmeyer, Holguin, & Mezulis, 2017). Studies of survivors of natural disasters, including earth- quakes and volcanic eruptions, have also found associations been gratitude and post-traumatic growth (Lies, Mellor, & Hong, 2014) (Subandi, Achmad, Kurniati, & Febri, 2014) (Zhou & Wu, 2016).

Children may benefit from learning to cultivate gratitude in specific ways:

• Studies have found that more grateful adolescents and college students are more interested in school, perform better academically, have better interpersonal relationships, and are happier with their school experience (M. Ma, Kibler, & Sly, 2013)
• A led by Giacomo Bono, conducted as part of the ESPG’s Youth Gratitude Project, tracked adolescents for four years (from ages 10-14 to 14-18) and examined the relation- ship between gratitude and the development of prosocial behavior (Bono et al., 2017). It found that growth in gratitude predicted increased prosocial behavior, and this relationship was reciprocal: Increases in prosocial behavior also predicted increases in gratitude.
• Middle school students assigned to count blessings each school day for two weeks showed decreased negative affect and increased gratitude, optimism, life satisfaction, and satisfaction with school (Froh et al., 2008)
• Another study found that eight- to 19-year-old students who wrote a gratitude letter and read it to the person for whom they were grateful reported more gratitude and positive affect after the intervention and more positive affect two months later than students assigned to journal about their day, but only students who had low positive affect at the beginning of the study saw these positive outcomes (Froh, Kashdan, Ozimkowski, & Miller, 2009)

These comprehensive findings indicate in so many ways the value of positive thinking and statements, and the damage that comes from pessimistic attitudes and statements.