The pursuit of happiness has long stood at the crossroads of philosophy, psychology, and human aspiration. From Aristotle’s eudaimonia to modern psychology’s focus on subjective well-being, the central question endures: what constitutes genuine happiness, and how can it be sustained? Despite substantial advances, happiness remains a complex, multilayered construct.

Contemporary psychology distinguishes experiential forms of happiness the fleeting but powerful moments of pleasure, enjoyment, excitement, or gratification from more enduring dimensions such as life satisfaction and inner contentment [1-3]. Scholars have further argued that an abundance of choice can paradoxically reduce happiness by amplifying regret and uncertainty [4], while errors in affective forecasting hinder our ability to predict what truly sustains well-being [5].

While much of this literature emphasizes either momentary experiences or global evaluations, less attention has been paid to the interplay between external conditions, experiential states, and inner resilience. To address this gap, we previously developed a layered framework the Shell and Core Model of Subjective Well-Being that organizes happiness into an outer layer of protective “armor,” a shell of experiential states, and an inner core of stability [6]. That work provided empirical grounding across diverse cultural settings.

The present article does not re-present empirical findings; rather, it expands the conceptual architecture of the model. We elaborate the categories of experiential happiness (achievement-based joy, need-based satisfaction, activity-based excitement) and situate them within the broader structure of inner happiness. We also develop the metaphor of the Happiness Suit of Armor (HSOA), including traits such as flexibility, health, meaning, and connection that buffer individuals against life’s challenges. By reframing the model descriptively, we aim to offer a comprehensive conceptual map that integrates transient experiences with enduring states and situates happiness as both a psychological phenomenon and a determinant of health.

Ancient Philosophical Roots

The concept of happiness has been central to human thought for millennia. In ancient philosophy, Aristotle’s notion of eudaimonia emphasized a life of virtue and self-realization, distinguishing it from fleeting pleasure (hedonia) [7]. Epicurus, in contrast, argued that happiness derives from the moderation of desire and the pursuit of tranquility rather than indulgence [8]. These classical formulations established a lasting dichotomy between virtue-based and pleasure-based accounts of happiness.

Enlightenment and Utilitarian Approaches

During the Enlightenment, utilitarian thinkers reframed happiness as the maximization of pleasure and minimization of pain for the greatest number. Jeremy Bentham advanced the idea of a “hedonic calculus,” while John Stuart Mill refined it by distinguishing between higher and lower forms of happiness [9,10]. This era underscored the social and collective dimensions of happiness, linking individual well-being to broader moral and political considerations.

Early Psychological Models

In the 20th century, psychology began to treat happiness as a measurable construct. Abraham Maslow’s hierarchy of needs positioned self-actualization at the apex of human motivation, implying that happiness emerges as more basic physiological and safety needs are met [11]. Later, Ed Diener and colleagues introduced the concept of subjective well-being (SWB), encompassing both affective balance (positive vs. negative affect) and cognitive life evaluations [2,12]. Daniel Kahneman distinguished between experienced well-being, measured in real time through momentary reports, and evaluative well-being, based on retrospective judgments of life satisfaction [1].

The Positive Psychology Movement

The advent of positive psychology further expanded this field. Martin Seligman proposed that happiness rests on cultivating positive emotions, engagement, and meaning, later formalized as the PERMA model [3]. Barry Schwartz [4] highlighted the “paradox of choice,” showing that an excess of options can undermine well-being, while Daniel Gilbert [5] explored systematic errors in “affective forecasting.” Collectively, these contributions shifted the focus from pathology to flourishing, positioning happiness as a legitimate object of scientific inquiry.

The Gap in Existing Models

Despite these advances, a gap remains between models that emphasize fleeting experiential states (pleasure, enjoyment, excitement) and those that emphasize enduring dispositions (contentment, satisfaction, life purpose). It is within this context that the Shell and Core Model [6] was developed, aiming to integrate transient experiences with stable inner resources while highlighting the protective mechanisms that sustain well-being.

Components of the Shell and Core Model of Happiness

The Shell and Core Model [6] organizes happiness into three interdependent layers (Figure 1): (1) the outer armor, representing protective resources; (2) the shell, reflecting transient experiential states of happiness; and (3) the core, denoting inner stability and enduring well-being. This layered framework emphasizes not only how happiness is experienced but also how it is safeguarded and sustained across the fluctuations of life.

Figure 1: Shell and Core (S&C) model of subjective well-being, showing the shell (experiential happiness), the core (inner happiness), and the happiness suit of armor (HSOA) as protective traits.

The Shell (Experiential Happiness)

In the S&C model, any form of happy feeling that is a result of experiences with the surrounding world is considered to be situated within the shell of happiness. These experiences are categorized under three main domains, namely achievement-based happiness (joy), need-based happiness (satisfaction), and activity-based happiness (excitement). All forms of experiential happiness are time bound and result in generation of happy moments. However, the durability of each happy feeling is highly variable depending on the type of experience (Figure 2).

Figure 2: Defining various forms of happiness in the Shell & Core model.

The Structure of the Shell

Joy (Achievement-Based Happiness): Achievement-based happiness is the happy feeling associated with “having what one wants when one wants it,” whether it is a simple experience such as a bite of delicious food or a profound life-altering achievement. This form of happy feeling is further subdivided into three main subcategories of pleasure, enjoyment, and gratification.

• Pleasure:Pleasure is the most well-known and common type of joy, inseparable from being human. The feeling of pleasure is the result of simple sensory experiences such as taste, smell, vision, hearing, touch, or a complex combination of these sensory inputs, such as eating a meal or a sexual act. Pleasure is the most primitive form of happy feeling, and human as well as animal brains are hardwired to receive pleasure since birth. As a result, the desire to repeat the pleasurable activities may lead to habit formation or even addiction.
• Enjoyment:Enjoyment is a form of joy that can be experienced only after acquiring intellectual and interpersonal skills and by partaking in social interactions. As it places greater demand on one’s cognition, enjoyment engages thinking and learning, as well as communication skills. Hanging out with a friend, reading a book or a poem, watching a movie, and having a pleasant conversation are all examples of enjoyment. Enjoyment provides a deeper and more meaningful sense of happiness in comparison with pleasure, which only entails a simple engagement of our senses. The happy feeling associated with enjoyment is more durable than pleasure since it strikes the chord harder with one’s true self. Moreover, it does not require external input, as positive emotions can be experienced through meditation. In particular, mindful meditation, which requires an individual to refrain from any physical or mental activities, and to eliminate all social and environmental inputs, gives rise to thought-less moments that can be experienced as pure happiness. This practice is not only one of the best methods to diffuse stress by allowing the mind to reboot itself but also provides the opportunity for the practicing individual to be present in the moment. Even though the happy feeling associated with enjoyment is more amenable to reasoning, people can make a habit of enjoyable activities or fall prey to addiction.
• Gratification:Gratification is a form of joy that stems from the approval and appreciation of others, which can range from a simple verbal compliment for a good deed to a widespread recognition for receiving a lifetime achievement or award. This form of experiential happiness is typically sought by those with more extensive social interactions and greater intellectual capabilities who derive their happiness from that of others. These individuals typically engage in activities that benefit their community, work on building a better world, or strive to make a meaningful difference in the life of others. As gratification is the highest level of joy, the happy feeling associated with this form of achievement-based happiness is the most durable. The impact of experiencing a meaningful gratification is usually life-changing due to which this experience is far more memorable than other forms of experiential happiness.

While this simple subdivision is rooted in empirical evidence, it is not easy to categorize most of the real-life experiences into one of the aforementioned achievement-based types. For example, the experience of going out for dinner with a friend could result in a mixture of pleasure derived from eating a good food, enjoyment of socializing with a friend, and gratification arising from the friend’s appreciation. Indeed, most of the daily routine activities can give rise to a mixture of different forms of achievement-based happiness.

Satisfaction (Need-Based Happiness): Satisfaction as a form of happy feeling in response to one’s needs is the other form of experiential happiness that starts at birth. The need-based happiness peaks when the need is met. We experience such satisfaction on a daily basis as the pleasant feeling of satiety after a period of hunger or the sense of relief after going to the bathroom after a long travel. In the S&C model, however, satisfaction also extends to the positive emotions associated with fulfilling an obligation.

Excitement (Activity-Based Happiness): Excitement is yet another form of experiential happiness that is associated with activities, desired physical or mental activities in particular. An extreme form of excitement arises when the individual is fully committed and involved in a creative activity that matches their skills and knowledge, giving rise to the concept of “state of flow” [13]. Many believe that state of flow is the ultimate form of happiness, which can be attained with or without having a set goal. For example, activities such as playing an instrument, playing a game, or doing exercise could all have positive effects on the individual and result in a happy feeling. When engaging in these activities provides thought-less moments away from the surrounding distractions, they can also provide a means for diffusing stress.

Overlap and Temporality: These categories often overlap; for example, the joy of achievement may also generate satisfaction or excitement (Figure 3). What unites them is their temporality. Experiential happiness builds in anticipation, peaks during the event, and fades over time. Repetition diminishes its impact due to habituation, the natural decline in response to familiar stimuli.

Evolution across the Lifespan: Experiential happiness also evolves over time. Freud suggested that individuals “outgrow” happiness as they age. In contrast, the S&C model proposes that happiness does not diminish but changes form. Early life is dominated by pleasure and excitement, whereas later life often emphasizes satisfaction, meaning, and contribution. The metaphor of a seed growing into a tree illustrates this process: pleasure sparks early growth, but with maturity, happiness derives increasingly from offering “shade” to others.

Habit Formation and Addiction: The repetition of happy moments makes experiential happiness habit-forming. While such repetition often enriches life, it also carries risks. When the pursuit of pleasure or gratification becomes compulsive, it may devolve into addiction. In the S&C framework, addiction represents a distortion in which achievement-based or activity-based happiness shifts into a need-based form, where compulsion replaces joy. This reflects the Duality of Reasons, where obligations dominate choice and happiness is diverted into maladaptive cycles.

Money and the Shell of Happiness: Income exerts a notable but limited influence on experiential happiness. Research shows that while income strongly predicts well-being at lower levels, the effect plateaus once basic needs are met [14]. In the S&C model, money can enhance nearly all forms of shell-level happiness:

• Need-based happinesscan be purchased by meeting necessities.
• Achievement-based happinesscan be amplified through material pleasures, social experiences, and public recognition.
• Activity-based happinesscan be supported when financial security enables immersion in flow-generating pursuits.

Yet money does not strengthen the armor or deepen the core. This explains why decades of rising income have not translated into greater overall happiness at the societal level.

Figure 3: Overlapping domains of experiential happiness within the Shell & Core framework.

The Core: Inner Happiness

At the center lies the core of happiness, representing inner contentment. Unlike experiential happiness, which depends on external triggers, inner happiness is self-sustaining and relatively stable. It functions as the foundation upon which happy moments are built: if inner happiness is the cake, experiential happiness is the icing. Without a stable cake, even the most abundant icing cannot satisfy.

Stability and Determinants

Inner happiness provides a baseline of well-being that endures over time. Genetic factors account for a substantial portion of individual differences, with twin studies suggesting that up to 40–50% of variance in happiness may be heritable [15]. Childhood experiences and upbringing also contribute significantly, shaping enduring personality traits and coping styles that sustain contentment throughout life.

The Happiness Suit of Armor (HSOA)

The S&C model of SWB is a comprehensive model based on a combination of eudaimonic and hedonistic perspectives. It considers a wide range of happy feelings affected by external circumstances in the shell of happiness (such as achievements, satisfaction of needs, and engagement in physical and mental activities) as well as core of happiness (supplying inner happiness), and an interface layer linking the shell and core of happiness (named HSOA) involving psychological characteristics pertaining to coping and defense mechanisms. Such a modular framework allows any possible future expansion or adjustment to the model to ensure that it remains responsive to the changing social reality. Moreover, as the model variables are based on current data obtained from this preliminary research and can be modified and rearranged based on more complementary larger studies and be applied in a variety of clinical settings.

In its development, SWB was conceptualized as a multitude of subjective experiences that can only be expressed through words. Therefore, it was assumed that, based on the theory of linguistic relativity, one’s worldview and overall perception of one’s external environment is influenced by the structure of their primary language. This issue is further compounded by the fact that happiness is difficult to describe precisely, as it is often equated with joy, pleasure, exhilaration, bliss, contentedness, delight, felicity, enjoyment, and satisfaction, even though each of these sensations is a distinct phenomenon. Thus, to avoid any ambiguity, in the S&C model, specific words were assigned to different contexts that may give rise to happy feelings. Even though this assignment could be considered arbitrary, it was necessary for defining different forms of happy feeling related to various physical, mental, social, and spiritual experiences. Moreover, providing a common terminology makes the discussion of various real-life applications of SWB much easier. Therefore, the S&C model not only provides a framework for defining various forms of happy feelings, it also provides insight into the interactions among these factors that give rise to SWB.

• Helmet-Knowledge and Optimism:Protects the mind by framing challenges as opportunities. Knowledge broadens perspective, and optimism sustains hope in uncertainty.
• Chest-Love and Emotion:Emotional center where giving and receiving love provides warmth, belonging, and resilience against despair [16].
• Abdomen-Management (Discipline and Spontaneity):Represents the balance between regulation and freedom, allowing effective organization while retaining flexibility.
• Back-Health and Success:A strong back symbolizes physical vitality, supporting resilience against life’s burdens.
• Right Hand-Trust and Flexibility:Extends outward in cooperation, enabling adaptability and meaningful relationships [17].
• Left Hand-Support and Generosity:Symbolizes service to others, reinforcing reciprocal social bonds and collective well-being.
• Right Foot-Freedom and Loyalty:Provides mobility and autonomy, grounding happiness in self-determination.
• Left Foot-Peace and Presence:Anchors the self in calmness, purpose, and spiritual grounding [18].
• Shield-Confidence and Humility:Defends against self-doubt and external negativity while balancing pride with modesty.
• Sword-Justice and Forgiveness:Defends values while promoting reconciliation; together they prevent bitterness and protect long-term well-being.

This mapping illustrates that high subjective well-being depends not only on transient joys in the shell but also on the stability of inner contentment in the core, reinforced by the protective influence of HSOA traits.

Although depicted as distinct, the shell and core are dynamically interconnected.

• Core → Shell: Inner happiness shapes how outer moments are experienced. For example, the shift from achievement-based happiness (“getting what we want”) to acquired happiness (“wanting what we have”) demonstrates how contentment reframes experience, reducing the endless pursuit of more (Figure 4). This may explain why highly affluent nations are not necessarily the happiest.
• Shell → Core: Positive experiences can strengthen the armor and, indirectly, the core itself. A temporary joy, such as travel or accomplishment, may extend beyond the moment by enhancing coping skills, strengthening relationships, or reinforcing perspective thereby enriching inner happiness over time.
• The Cumulative Properties of Positive Affect: Ample body of evidence shows that positive affect has a cumulative beneficial influence on general SWB and success in life [19]. The collective effect of positive emotions is also the basis for broaden-and-build theory of happiness [20]. Accordingly, the S&C model of SWB postulates that the core of happiness can be expanded with frequent positive reinforcement of the shell of happiness, helping the individual weather life experiences associated with negative affect. The model further predicts that the effect of frequently adopting a positive attitude also has the capacity to strengthen the HSOA and its buffering action to mitigate the negative effect of negative life experiences.

Figure 4: Overall levels of happiness reported by study participants.

Many believe that happiness is the ultimate goal in life [21], even though a recent study did not support this notion [22]. Nonetheless, as happiness is very important to most individuals, numerous attempts have been made to create a unifying model for defining and understanding happiness. These models incorporate a variety of internal and external factors that have been shown to play a role in one’s positive emotions and happiness. Happiness is not merely a subjective state of mind; it exerts a profound influence on physical health and disease outcomes. A growing body of evidence demonstrates that well-being contributes to resilience against illness, recovery from disease, and overall longevity [23]. Conversely, chronic unhappiness, stress, and negative affect have been linked to a broad spectrum of medical conditions, including cardiovascular disease, immune dysfunction, and gastrointestinal disorders [24,25].

The Gut-Brain Connection

One particularly important interface between happiness and health lies in the gut brain axis. Emotional states directly affect gastrointestinal physiology through neural, endocrine, and immune pathways. In earlier work, we explored this relationship under the concept of the “gut feeling” [26], demonstrating that psychological well-being and gastrointestinal symptoms are closely intertwined. This research highlighted that disturbances in happiness and emotional regulation can manifest as gastrointestinal complaints, while gut dysfunction in turn exacerbates negative emotional states establishing a bidirectional loop.

Psychosomatic and Systemic Effects

The influence of happiness extends beyond the gastrointestinal tract. Positive affect and psychological well-being are associated with lower levels of systemic inflammation, improved immune response, and reduced risk of chronic disease [24]. For example, individuals with higher baseline happiness demonstrate better cardiovascular outcomes and even increased longevity [27]. These findings reinforce the clinical observation that patients with greater contentment and life satisfaction tend to tolerate illness better and recover more rapidly.

Within the Shell and Core framework, these observations suggest that both shell-level experiences and core-level contentment play roles in physical health. Transient experiences of joy and satisfaction can reduce acute stress responses, while enduring inner happiness provides a stable baseline that mitigates long-term physiological wear and tear. Moreover, the Happiness Suit of Armor (HSOA) with its components of health, flexibility, connection, and meaning functions not only as psychological protection but also as a biological buffer against disease vulnerability.

By recognizing the interplay between happiness and health, the S&C model underscores the importance of promoting well-being as part of comprehensive medical care [28-32]. Happiness is not only an outcome of good health but also a determinant of health, shaping how disease is experienced, coped with, and even biologically mediated.

Implications of the Shell and Core Model

The Shell and Core (S&C) Model provides a framework that extends beyond traditional approaches to subjective well-being by integrating experiential states, inner stability, and protective resources. Its implications span several domains:

• Psychological Theory: The model bridges hedonic and eudaimonic traditions by situating transient joy (shell) within the context of enduring contentment (core) and protective traits (armor) [33-35]. It underscores that happiness is not reducible to either fleeting pleasure or life satisfaction but arises from their dynamic interaction.
• Clinical Practice: By incorporating the Happiness Suit of Armor (HSOA), the model highlights protective traits that can be strengthened through therapeutic interventions. Cultivating optimism, flexibility, meaning, and social connection may buffer individuals against stress, anxiety, and depression, while reinforcing long-term well-being.
• Health and Medicine: The S&C model emphasizes that happiness influences not only mental health but also physical health. Evidence from gut–brain research, immune function, and cardiovascular studies demonstrates that subjective well-being can shape disease vulnerability and recovery [36,37]. Integrating happiness into clinical care may thus improve outcomes across multiple domains of health.
• Education and Development: The layered approach offers a useful tool for educational settings, where promoting not only achievement but also resilience and contentment can enhance student well-being and life satisfaction.
• Public Policy and Society: On a societal level, the model suggests that policy should not only aim to raise income or provide opportunities for pleasure but also cultivate environments that nurture connection, meaning, health, and inner stability. This broader perspective aligns with global initiatives that emphasize well-being as a measure of national progress.

Although the Shell and Core Model offers a comprehensive framework, several limitations must be acknowledged.

• Conceptual Nature: As presented here, the model is descriptive and integrative rather than empirically tested in full. While prior survey work has provided initial support, further research is required to validate the distinct contributions of shell, core, and armor to overall well-being.
• Cultural Variability: Happiness is shaped by cultural values and societal norms. While the S&C model aims to be universal, its components (e.g., autonomy, flow, or satisfaction) may differ in salience across cultural contexts. Future cross-cultural studies are necessary to assess the model’s generalizability.
• Measurement Challenges: Operationalizing the components of the armor and differentiating experiential from inner happiness present methodological difficulties. Developing validated instruments to capture these dimensions will be critical for advancing research.
• Dynamic Interactions: The interactions between shell, core, and armor are hypothesized to be bidirectional, but their causal pathways remain unclear. Longitudinal studies are needed to clarify how positive experiences strengthen the core and how contentment influences the perception of external events.
• Integration with Neuroscience: While the model resonates with findings from gut–brain and affective neuroscience research, its neural correlates remain to be established. Future work could integrate the S&C framework with neuroimaging, psychophysiology, and biological markers of stress and resilience.

Advancing the S&C model will require a combination of empirical validation, cross-cultural exploration, and clinical application. Longitudinal and intervention-based studies could test whether strengthening armor traits enhances both experiential and inner happiness. Cross-disciplinary collaborations bridging psychology, medicine, neuroscience, and public policy may also deepen understanding of happiness as a determinant of human flourishing and health.

The Shell and Core Model of Happiness reframes well-being as a layered and dynamic system composed of transient experiential states, a stable foundation of inner contentment, and a protective suit of psychological armor. By integrating perspectives from philosophy, psychology, and medicine, this framework highlights how happiness is not a singular construct but an interplay of immediate joy, enduring stability, and resilient coping.

The shell captures the richness of experiential happiness pleasure, satisfaction, and excitement while acknowledging its temporality, evolution across the lifespan, and vulnerability to habit or addiction. The corerepresents inner contentment, a stable baseline shaped by genetics, upbringing, and life perspective, upon which momentary experiences rest. Between them, the Happiness Suit of Armor illustrates the protective traits that buffer adversity and sustain well-being, ranging from optimism and love to flexibility, health, and forgiveness.

Crucially, happiness is not only a psychological construct but also a determinant of health. Evidence shows that well-being influences systemic physiology, from immune function to cardiovascular outcomes, while disturbances in happiness contribute to chronic disease and psychosomatic symptoms. Prior work on the gut–brain axis reinforces this link, demonstrating how happiness and gastrointestinal health are bidirectionally connected.

Taken together, the Shell and Core Model provides a conceptual framework that extends beyond measuring life satisfaction or fleeting moments of joy. It situates happiness as a multi-layered, evolving system at once experiential, protective, and foundational. By recognizing this complexity, researchers, clinicians, and policymakers can better address the cultivation and protection of happiness, not merely as a luxury of life, but as a cornerstone of human flourishing and health.

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